diff --git a/packages/backend/src/core/CustomEmojiService.ts b/packages/backend/src/core/CustomEmojiService.ts
index 17f744928119..145c224f678a 100644
--- a/packages/backend/src/core/CustomEmojiService.ts
+++ b/packages/backend/src/core/CustomEmojiService.ts
@@ -379,6 +379,20 @@ export class CustomEmojiService implements OnApplicationShutdown {
 		}
 	}
 
+	/**
+	 * ローカル内の絵文字に重複がないかチェックします
+	 * @param name 絵文字名
+	 */
+	@bindThis
+	public checkDuplicate(name: string): Promise<boolean> {
+		return this.emojisRepository.exist({ where: { name, host: IsNull() } });
+	}
+
+	@bindThis
+	public getEmojiById(id: string): Promise<MiEmoji | null> {
+		return this.emojisRepository.findOneBy({ id });
+	}
+
 	@bindThis
 	public dispose(): void {
 		this.cache.dispose();
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 24d3a8a94396..faab8ee608da 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/add.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/add.ts
@@ -23,6 +23,11 @@ export const meta = {
 			code: 'NO_SUCH_FILE',
 			id: 'fc46b5a4-6b92-4c33-ac66-b806659bb5cf',
 		},
+		duplicateName: {
+			message: 'Duplicate name.',
+			code: 'DUPLICATE_NAME',
+			id: 'f7a3462c-4e6e-4069-8421-b9bd4f4c3975',
+		},
 	},
 } as const;
 
@@ -64,6 +69,8 @@ export default class extends Endpoint<typeof meta, typeof paramDef> { // eslint-
 		super(meta, paramDef, async (ps, me) => {
 			const driveFile = await this.driveFilesRepository.findOneBy({ id: ps.fileId });
 			if (driveFile == null) throw new ApiError(meta.errors.noSuchFile);
+			const isDuplicate = await this.customEmojiService.checkDuplicate(ps.name);
+			if (isDuplicate) throw new ApiError(meta.errors.duplicateName);
 
 			const emoji = await this.customEmojiService.add({
 				driveFile,
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 2d69857408ef..04226d895389 100644
--- a/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
+++ b/packages/backend/src/server/api/endpoints/admin/emoji/update.ts
@@ -74,6 +74,15 @@ 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 emoji = await this.customEmojiService.getEmojiById(ps.id);
+			if (emoji != null) {
+				if (ps.name !== emoji.name) {
+					const isDuplicate = await this.customEmojiService.checkDuplicate(ps.name);
+					if (isDuplicate) throw new ApiError(meta.errors.sameNameEmojiExists);
+				}
+			} else {
+				throw new ApiError(meta.errors.noSuchEmoji);
+			}
 
 			await this.customEmojiService.update(ps.id, {
 				driveFile,