Update 7project/backend/app/api/transactions.py

Better error message

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

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)