If you accidentally entered a sales invoice twice in your March GSTR-3B return, you can correct this by reducing the sales amount in your April return. While this might create a temporary mismatch, it's permissible. Alternatively, you can report the correct sales figures in your Annual Return, which may lead to an excess tax payment situation requiring a refund application.
01 August 2020
In my view, you should file a refund for excess payment of tax inadvertently. In Annual return if you show correct sales (which you have to) then you will have a situation of excess payment of tax.