diff --git a/7project/backend/app/api/transactions.py b/7project/backend/app/api/transactions.py
index 8a7215d..5fc361c 100644
--- a/7project/backend/app/api/transactions.py
+++ b/7project/backend/app/api/transactions.py
@@ -44,7 +44,10 @@ async def create_transaction(
         )
         categories = list(res.scalars())
         if len(categories) != len(set(payload.category_ids)):
-            raise HTTPException(status_code=400, detail="One or more categories not found")
+            raise HTTPException(
+                status_code=400,
+                detail="Duplicate category IDs provided or one or more categories not found"
+            )
         tx.categories = categories
 
     session.add(tx)
@@ -113,13 +116,16 @@ async def update_transaction(
         # Preload categories to avoid async lazy-load during assignment
         await session.refresh(tx, attribute_names=["categories"])
         if payload.category_ids:
+            # Check for duplicate category IDs in the payload
+            if len(payload.category_ids) != len(set(payload.category_ids)):
+                raise HTTPException(status_code=400, detail="Duplicate category IDs in payload")
             res = await session.execute(
                 select(Category).where(
                     Category.user_id == user.id, Category.id.in_(payload.category_ids)
                 )
             )
             categories = list(res.scalars())
-            if len(categories) != len(set(payload.category_ids)):
+            if len(categories) != len(payload.category_ids):
                 raise HTTPException(status_code=400, detail="One or more categories not found")
             tx.categories = categories
         else: