how to delete an entry which had been posted

You can remove a duplicate entry in QBO by undoing it from the Posted tab in Bank transactions, then excluding it. Here's how:

1. Go to Bank transactions.
2. Click the Posted tab.
3. Find the duplicate entry you want to remove.
4. In the Action column, click Undo. This sends the transaction back to pending.
5. Select the Pending tab and locate the transaction you just undid.
6. In the Action column, click the dropdown, then select Exclude.

For more details, you can refer to this article: Remove duplicate bank transactions in QuickBooks Online.

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