diff --git a/packages/toolkit/src/mapBuilders.ts b/packages/toolkit/src/mapBuilders.ts
--- a/packages/toolkit/src/mapBuilders.ts
+++ b/packages/toolkit/src/mapBuilders.ts
@@ -166,7 +166,7 @@ export function executeReducerBuilderCallback<S>(
       }
       if (type in actionsMap) {
         throw new Error(
-          '`builder.addCase` cannot be called with two reducers for the same action type'
+          `\`builder.addCase\` cannot be called with two reducers for the same action type '${type}'`
         )
       }
       actionsMap[type] = reducer

diff --git a/packages/toolkit/src/tests/createReducer.test.ts b/packages/toolkit/src/tests/createReducer.test.ts
--- a/packages/toolkit/src/tests/createReducer.test.ts
+++ b/packages/toolkit/src/tests/createReducer.test.ts
@@ -460,7 +460,7 @@ describe('createReducer', () => {
             .addCase(decrement, (state, action) => state - action.payload)
         )
       ).toThrowErrorMatchingInlineSnapshot(
-        '"`builder.addCase` cannot be called with two reducers for the same action type"'
+        '"`builder.addCase` cannot be called with two reducers for the same action type \'increment\'"'
       )
       expect(() =>
         createReducer(0, (builder) =>
@@ -470,7 +470,7 @@ describe('createReducer', () => {
             .addCase(decrement, (state, action) => state - action.payload)
         )
       ).toThrowErrorMatchingInlineSnapshot(
-        '"`builder.addCase` cannot be called with two reducers for the same action type"'
+        '"`builder.addCase` cannot be called with two reducers for the same action type \'increment\'"'
       )
     })
 

Clarifying error message about type in addCase
I am newbie in RTK and when I met error `'builder.addCase' cannot be called with two reducers for the same action type` I cannot recognize which one of my types is duplcated.
I solved my problem with devtools logpoint but it's not convenient at all
![image](https://github.com/reduxjs/redux-toolkit/assets/1235286/45847738-0573-40de-a5d6-48f59e053028)
![image](https://github.com/reduxjs/redux-toolkit/assets/1235286/8278f088-a35d-49e6-bf25-540d2dd9258c)

It would be great if error will contain type. For example:
`'builder.addCase' cannot be called with two reducers for the same action type ${type}`

Sure, makes sense. Can you file a PR to tweak that?
@markerikson Sure!