++ Incrementing by 2 instead of 1 in React JS

Question

The code below is supposed to increment & decrement the quantity by 1 but instead, it is incrementing and decrementing by 2

switch(action.payload) {
  case "INC_QUANTITY": {
        let newState = state;
        newState[action.payload].quantity += 1;
        return [...newState];
      }
      case "DEC_QUANTITY": {
        let newState = state;
        newState[action.payload].quantity--;
        return [...newState];
      }
}

newState is an array of objects and each object has its own quantity key, action.payload is the index of object which quantity key value is supposed to be increased by 1 example of object is

const newState = [
  {
    category: "Female",
    desc: "Slim fit regular blue jeans for women",
    id: "DyDlMp_7hi",
    name: "Blue Jeans",
    price: 25,
    quantity: 1,
    url: "img_url",
  }
]

I am using react context API and useContext to transfer data between components and useReducer to manage the state have a look at the code below and you will have an understanding

const [mensProductState, menDispatch] = useReducer(reducer, [...menProducts]);
  const [ladiesProduct, ladiesDispatch] = useReducer(reducer, [...feminineProducts]);
  const [newArrivalState, newArrivalDispatch] = useReducer(reducer, [...newArrival]);
  const [cartState, cartDispatch] = useReducer(cartReducer, []);
  const value = useMemo(
    () => ({
      cartState,
      cartDispatch,
      ladiesProduct,
      ladiesDispatch,
      mensProductState,
      menDispatch,
      newArrivalState,
      newArrivalDispatch,
    }),
    [cartState, ladiesProduct, mensProductState, newArrivalState]
  );
  return (
    <StoreContext.Provider value={value}>
      <BrowserRouter>
        <Routes>
          <Route index element={<Home />} />
          <Route path="/cart" element={<Cart />} />
        </Routes>
      </BrowserRouter>
    </StoreContext.Provider>

Below is the SnapShot of the Application

plus icon is supposed to increment the quantity