diff --git a/backend/package.json b/backend/package.json
index 8f0628d..b8a4365 100644
--- a/backend/package.json
+++ b/backend/package.json
@@ -1,6 +1,6 @@
 {
   "name": "backend",
-  "version": "1.12.0",
+  "version": "1.12.1",
   "private": true,
   "type": "commonjs",
   "main": "src/index.js",
diff --git a/backend/src/routes/api.integration.test.js b/backend/src/routes/api.integration.test.js
index 68c000e..44ef92d 100644
--- a/backend/src/routes/api.integration.test.js
+++ b/backend/src/routes/api.integration.test.js
@@ -1454,6 +1454,12 @@ describe('default content library', () => {
     expect(addRes.body.weapon.name).toBe('Longsword');
     expect(addRes.body.weapon.campaignId).toBe(campaign.id);
 
+    // Each library entry goes in once, until its copy is renamed or deleted.
+    const addLongsword = () => request(app).post(`/campaigns/${campaign.id}/weapons/from-library`).set(dmAuth).send({ key: 'longsword' });
+    expect((await addLongsword()).status).toBe(409);
+    await request(app).delete(`/campaigns/${campaign.id}/weapons/${addRes.body.weapon.id}`).set(dmAuth);
+    expect((await addLongsword()).status).toBe(201);
+
     // Library spells show their level and default damage before they are added,
     // and carry both into the campaign.
     const byKey = Object.fromEntries(listRes.body.spells.map((s) => [s.key, s]));
diff --git a/backend/src/routes/campaigns.routes.js b/backend/src/routes/campaigns.routes.js
index cc78042..ecff411 100644
--- a/backend/src/routes/campaigns.routes.js
+++ b/backend/src/routes/campaigns.routes.js
@@ -948,6 +948,14 @@ router.delete('/:id/wares/:wareId', requireCampaignMember(), requireDM(), async
 // backend/src/game/data/{weapons,spells,monsters}Library.js for the sets.
 // ---------------------------------------------------------------------------
 
+// A library entry goes into a campaign once: it is taken for as long as the
+// pool holds a row by its name, and free again when that row is renamed or
+// deleted.
+async function inPool(model, campaignId, name) {
+  const row = await prisma[model].findFirst({ where: { campaignId, name: { equals: name, mode: 'insensitive' } }, select: { id: true } });
+  return Boolean(row);
+}
+
 router.get('/:id/library', requireCampaignMember(), requireDM(), async (req, res, next) => {
   try {
     res.json({ weapons: WEAPON_LIBRARY, items: ITEM_LIBRARY, spells: SPELL_LIBRARY, monsters: MONSTER_LIBRARY });
@@ -962,6 +970,9 @@ router.post('/:id/weapons/from-library', requireCampaignMember(), requireDM(), a
     if (!parsed.success) return res.status(400).json({ error: parsed.error.flatten() });
     const template = WEAPON_LIBRARY.find((w) => w.key === parsed.data.key);
     if (!template) return res.status(404).json({ error: 'Unknown library weapon.' });
+    if (await inPool('weapon', req.campaign.id, template.name)) {
+      return res.status(409).json({ error: `${template.name} is already in this campaign.` });
+    }
     const { key, category, ...data } = template;
     const weapon = await prisma.weapon.create({ data: { ...data, campaignId: req.campaign.id } });
     res.status(201).json({ weapon });
@@ -976,6 +987,9 @@ router.post('/:id/items/from-library', requireCampaignMember(), requireDM(), asy
     if (!parsed.success) return res.status(400).json({ error: parsed.error.flatten() });
     const template = ITEM_LIBRARY.find((i) => i.key === parsed.data.key);
     if (!template) return res.status(404).json({ error: 'Unknown library item.' });
+    if (await inPool('item', req.campaign.id, template.name)) {
+      return res.status(409).json({ error: `${template.name} is already in this campaign.` });
+    }
     const { key, category, ...data } = template;
     const item = await prisma.item.create({ data: { ...data, campaignId: req.campaign.id } });
     res.status(201).json({ item });
@@ -990,6 +1004,9 @@ router.post('/:id/spells/from-library', requireCampaignMember(), requireDM(), as
     if (!parsed.success) return res.status(400).json({ error: parsed.error.flatten() });
     const template = SPELL_LIBRARY.find((s) => s.key === parsed.data.key);
     if (!template) return res.status(404).json({ error: 'Unknown library spell.' });
+    if (await inPool('spell', req.campaign.id, template.name)) {
+      return res.status(409).json({ error: `${template.name} is already in this campaign.` });
+    }
     const { key, classes, ...data } = template;
     const spell = await prisma.spell.create({ data: { ...data, campaignId: req.campaign.id } });
     res.status(201).json({ spell });
@@ -1004,6 +1021,9 @@ router.post('/:id/monsters/from-library', requireCampaignMember(), requireDM(),
     if (!parsed.success) return res.status(400).json({ error: parsed.error.flatten() });
     const template = MONSTER_LIBRARY.find((m) => m.key === parsed.data.key);
     if (!template) return res.status(404).json({ error: 'Unknown library monster.' });
+    if (await inPool('monster', req.campaign.id, template.name)) {
+      return res.status(409).json({ error: `${template.name} is already in this campaign.` });
+    }
     const { key, ...data } = template;
     const monster = await prisma.monster.create({ data: { ...data, campaignId: req.campaign.id } });
     res.status(201).json({ monster });
diff --git a/docs/ITEMS.md b/docs/ITEMS.md
index 8b0c5cd..c485227 100644
--- a/docs/ITEMS.md
+++ b/docs/ITEMS.md
@@ -53,7 +53,9 @@ The pool is the "Items Pool" card on the campaign page (DM only).
   worn gear such as a Ring of Protection, and common supplies. Each is copied
   into the pool with its effects already set (Chain Mail: +6 AC, −10 Speed,
   and a note about Stealth), at starting level 1 and not ticked as a starting
-  option. The copy is yours to change. The full list is in section 10.
+  option. The copy is yours to change. The full list is in section 10. Each
+  can be added once: an item the pool already has by that name is left out of
+  the library list, and comes back if the pool's item is renamed or deleted.
 - **Effects:** the "Effects" button on an item's row. The row shows what the
   item does under its name.
 - **Changing:** starting level, growth (when the item has dice) and the
diff --git a/docs/MONSTERS.md b/docs/MONSTERS.md
index 14f8ef8..2d7c30b 100644
--- a/docs/MONSTERS.md
+++ b/docs/MONSTERS.md
@@ -57,6 +57,9 @@ The list is the "Monsters" card on the campaign page (DM only).
   scores, special trait and notes. Only name, AC and HP must be typed; speed
   starts at 30, attack bonus at 0 and each ability score at 10.
 - **Adding from the bestiary:** "📚 From Library" copies a complete stat block.
+  Each monster can be added once: one the campaign already has by that name is
+  left out of the list, and comes back if the campaign's monster is renamed or
+  deleted.
 - **Changing:** "Edit" on a monster's row opens the same form filled in.
   Saving changes the template, and copies already in encounters show the new
   stat block straight away; their current HP is left alone.
diff --git a/docs/SPELLS.md b/docs/SPELLS.md
index 68a961d..c587a0e 100644
--- a/docs/SPELLS.md
+++ b/docs/SPELLS.md
@@ -107,6 +107,10 @@ one shown.
 A spell added from the library is **not** a starting option until ticked. The
 library has nothing above level 5; add higher spells by hand.
 
+Each library spell can be added once. A spell the list already has by that
+name is left out of the library list, and comes back if the campaign's spell
+is renamed or deleted.
+
 ## 4. Granting spells to characters
 
 On a character's sheet, the DM picks a spell from the list and clicks Grant.
diff --git a/docs/WEAPONS.md b/docs/WEAPONS.md
index 1efb3da..44246ae 100644
--- a/docs/WEAPONS.md
+++ b/docs/WEAPONS.md
@@ -86,6 +86,10 @@ growth, and **not** ticked as a starting option. Change those in the table
 afterwards. Every library weapon is listed in
 [section 12](#12-the-complete-weapon-library).
 
+Each library weapon can be added once. A weapon the pool already has by that
+name is left out of the library list, and comes back if the pool's weapon is
+renamed or deleted.
+
 ## 4. Giving weapons to characters
 
 On a character's sheet, the DM picks a weapon from the pool and clicks Give.
diff --git a/frontend/package.json b/frontend/package.json
index 76e086b..c1d26f3 100644
--- a/frontend/package.json
+++ b/frontend/package.json
@@ -1,7 +1,7 @@
 {
   "name": "frontend",
   "private": true,
-  "version": "1.12.0",
+  "version": "1.12.1",
   "type": "module",
   "scripts": {
     "dev": "vite",
diff --git a/frontend/src/pages/CampaignHomePage.tsx b/frontend/src/pages/CampaignHomePage.tsx
index 10448da..acb244d 100644
--- a/frontend/src/pages/CampaignHomePage.tsx
+++ b/frontend/src/pages/CampaignHomePage.tsx
@@ -503,6 +503,7 @@ function NpcTrade({
 
 function LibraryPicker<T extends { key: string; name: string }>({
   items,
+  taken,
   onAdd,
   adding,
   renderMeta,
@@ -510,6 +511,9 @@ function LibraryPicker<T extends { key: string; name: string }>({
   placeholder,
 }: {
   items: T[];
+  // What the campaign already holds: a library entry by one of these names is
+  // left out, since each can be added once.
+  taken: { name: string }[] | undefined;
   onAdd: (key: string) => void;
   adding: boolean;
   renderMeta: (item: T) => string;
@@ -518,7 +522,9 @@ function LibraryPicker<T extends { key: string; name: string }>({
   placeholder: string;
 }) {
   const [query, setQuery] = useState('');
-  const filtered = items.filter((i) => i.name.toLowerCase().includes(query.toLowerCase()));
+  const takenNames = new Set((taken ?? []).map((t) => t.name.toLowerCase()));
+  const available = items.filter((i) => !takenNames.has(i.name.toLowerCase()));
+  const filtered = available.filter((i) => i.name.toLowerCase().includes(query.toLowerCase()));
   const { pageItems, page, pageCount, prevPage, nextPage } = usePagination(filtered, 6);
 
   return (
@@ -537,7 +543,11 @@ function LibraryPicker<T extends { key: string; name: string }>({
             </button>
           </div>
         ))}
-        {filtered.length === 0 && <p style={{ color: 'var(--text-muted)' }}>No matches.</p>}
+        {filtered.length === 0 && (
+          <p style={{ color: 'var(--text-muted)' }}>
+            {available.length === 0 ? 'Everything in the library is already in this campaign.' : 'No matches.'}
+          </p>
+        )}
       </div>
       <Pagination page={page} pageCount={pageCount} onPrev={prevPage} onNext={nextPage} />
     </div>
@@ -1262,6 +1272,7 @@ export function CampaignHomePage() {
             {showWeaponLibrary && library && (
               <LibraryPicker<LibraryWeapon>
                 items={library.weapons}
+                taken={weapons}
                 adding={addWeaponFromLibrary.isPending}
                 onAdd={(key) => addWeaponFromLibrary.mutate(key)}
                 placeholder="Search the weapon library…"
@@ -1378,6 +1389,7 @@ export function CampaignHomePage() {
             {showItemLibrary && library && (
               <LibraryPicker<LibraryItem>
                 items={library.items}
+                taken={items}
                 adding={addItemFromLibrary.isPending}
                 onAdd={(key) => addItemFromLibrary.mutate(key)}
                 placeholder="Search the item library: armor, shields, worn gear, supplies…"
@@ -1496,6 +1508,7 @@ export function CampaignHomePage() {
             {showSpellLibrary && library && (
               <LibraryPicker<LibrarySpell>
                 items={library.spells}
+                taken={spells}
                 adding={addSpellFromLibrary.isPending}
                 onAdd={(key) => addSpellFromLibrary.mutate(key)}
                 placeholder="Search the spell library…"
@@ -1615,6 +1628,7 @@ export function CampaignHomePage() {
             {showMonsterLibrary && library && (
               <LibraryPicker<LibraryMonster>
                 items={library.monsters}
+                taken={monsters}
                 adding={addMonsterFromLibrary.isPending}
                 onAdd={(key) => addMonsterFromLibrary.mutate(key)}
                 placeholder="Search the monster library…"
diff --git a/package-lock.json b/package-lock.json
index d2979d2..a1c30a5 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,19 +1,19 @@
 {
   "name": "dnd-campaign-manager",
-  "version": "1.12.0",
+  "version": "1.12.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "dnd-campaign-manager",
-      "version": "1.12.0",
+      "version": "1.12.1",
       "workspaces": [
         "backend",
         "frontend"
       ]
     },
     "backend": {
-      "version": "1.12.0",
+      "version": "1.12.1",
       "dependencies": {
         "@prisma/adapter-pg": "^7.10.0",
         "@prisma/client": "^7.10.0",
@@ -32,7 +32,7 @@
       }
     },
     "frontend": {
-      "version": "1.12.0",
+      "version": "1.12.1",
       "dependencies": {
         "@tanstack/react-query": "^5.103.2",
         "axios": "^1.20.0",
diff --git a/package.json b/package.json
index e117cf2..ffb7b16 100644
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "dnd-campaign-manager",
-  "version": "1.12.0",
+  "version": "1.12.1",
   "private": true,
   "description": "D&D 5e campaign manager — monorepo root",
   "workspaces": [