diff --git a/packages/backend/src/server/api/endpoints/admin/emoji/add.ts b/packages/backend/src/server/api/endpoints/admin/emoji/add.ts
index 7bd920c312..afad538925 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/add.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/add.ts
@@ -5,7 +5,7 @@
 
 import { Inject, Injectable } from '@nestjs/common';
 import { Endpoint } from '@/server/api/endpoint-base.js';
-import type { DriveFilesRepository } from '@/models/_.js';
+import type { DriveFilesRepository, EmojisRepository } from '@/models/_.js';
 import { DI } from '@/di-symbols.js';
 import { CustomEmojiService } from '@/core/CustomEmojiService.js';
 import { ModerationLogService } from '@/core/ModerationLogService.js';
@@ -24,6 +24,11 @@ export const meta = {
 			code: 'NO_SUCH_FILE',
 			id: 'fc46b5a4-6b92-4c33-ac66-b806659bb5cf',
 		},
+		duplicationEmojiAdd: {
+			message: 'This emoji is already added.',
+      code: 'DUPLICATION_EMOJI_ADD',
+      id: 'mattyaski_emoji_duplication_error',
+		}
 	},
 } as const;
 
@@ -57,7 +62,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 	constructor(
 		@Inject(DI.driveFilesRepository)
 		private driveFilesRepository: DriveFilesRepository,
-
+		private emojisRepository: EmojisRepository,
 		private customEmojiService: CustomEmojiService,
 
 		private emojiEntityService: EmojiEntityService,
@@ -67,6 +72,16 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			const driveFile = await this.driveFilesRepository.findOneBy({ id: ps.fileId });
 			if (driveFile == null) throw new ApiError(meta.errors.noSuchFile);
 
+			const existEmoji = await this.emojisRepository.exist({
+				where: {
+					name: ps.name,
+				},
+			});
+
+			if (existEmoji) {
+				throw new ApiError(meta.errors.duplicationEmojiAdd);
+			}
+
 			const emoji = await this.customEmojiService.add({
 				driveFile,
 				name: ps.name,
@@ -79,6 +94,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				roleIdsThatCanBeUsedThisEmojiAsReaction: ps.roleIdsThatCanBeUsedThisEmojiAsReaction ?? [],
 			});
 
+
+
 			this.moderationLogService.insertModerationLog(me, 'addEmoji', {
 				emojiId: emoji.id,
 			});
diff --git a/packages/backend/src/server/api/endpoints/admin/emoji/copy.ts b/packages/backend/src/server/api/endpoints/admin/emoji/copy.ts
index f374b31303..5a592b989c 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/copy.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/copy.ts
@@ -26,6 +26,11 @@ export const meta = {
 			code: 'NO_SUCH_EMOJI',
 			id: 'e2785b66-dca3-4087-9cac-b93c541cc425',
 		},
+		duplicationEmojiAdd: {
+			message: 'This emoji is already added.',
+			code: 'DUPLICATION_EMOJI_ADD',
+			id: 'mattyaski_emoji_duplication_error',
+		}
 	},
 
 	res: {
@@ -57,6 +62,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		@Inject(DI.emojisRepository)
 		private emojisRepository: EmojisRepository,
 
+
 		private emojiEntityService: EmojiEntityService,
 		private idService: IdService,
 		private globalEventService: GlobalEventService,
@@ -77,6 +83,15 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 			} catch (e) {
 				throw new ApiError();
 			}
+			const existEmoji = await this.emojisRepository.exist({
+				where: {
+					name: emoji.name,
+				},
+			});
+
+			if (existEmoji) {
+				throw new ApiError(meta.errors.duplicationEmojiAdd);
+			}
 
 			const copied = await this.emojisRepository.insert({
 				id: this.idService.genId(),
diff --git a/packages/backend/src/server/api/endpoints/admin/emoji/update.ts b/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
index f01be9e27a..c8fb2a2a2d 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
@@ -6,7 +6,7 @@
 import { Inject, Injectable } from '@nestjs/common';
 import { Endpoint } from '@/server/api/endpoint-base.js';
 import { CustomEmojiService } from '@/core/CustomEmojiService.js';
-import type { DriveFilesRepository } from '@/models/_.js';
+import type { DriveFilesRepository , EmojisRepository } from '@/models/_.js';
 import { DI } from '@/di-symbols.js';
 import { ApiError } from '../../../error.js';
 
@@ -32,6 +32,11 @@ export const meta = {
 			code: 'SAME_NAME_EMOJI_EXISTS',
 			id: '7180fe9d-1ee3-bff9-647d-fe9896d2ffb8',
 		},
+		duplicationEmojiAdd: {
+			message: 'This emoji is already added.',
+			code: 'DUPLICATION_EMOJI_ADD',
+			id: 'mattyaski_emoji_duplication_error',
+		}
 	},
 } as const;
 
@@ -64,7 +69,7 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 	constructor(
 		@Inject(DI.driveFilesRepository)
 		private driveFilesRepository: DriveFilesRepository,
-
+		private emojisRepository: EmojisRepository,
 		private customEmojiService: CustomEmojiService,
 	) {
 		super(meta, paramDef, async (ps, me) => {
@@ -74,7 +79,15 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 				driveFile = await this.driveFilesRepository.findOneBy({ id: ps.fileId });
 				if (driveFile == null) throw new ApiError(meta.errors.noSuchFile);
 			}
+			const existEmoji = await this.emojisRepository.exist({
+				where: {
+					name: ps.name,
+				},
+			});
 
+			if (existEmoji) {
+				throw new ApiError(meta.errors.duplicationEmojiAdd);
+			}
 			await this.customEmojiService.update(ps.id, {
 				driveFile,
 				name: ps.name,