Quickbooks Error Code (503): Server Issue
If you a see a Duplicate Bill as a result, as long as the Bill is confirmed to be a duplicate, you can delete from QuickBooks Online.
This is a general error, that will almost always mean that Quickbooks Online is having server issues. This error will go away once Quickbooks has resolved this, unless there is another error for that item. In that case, resolving the other error will also resolve the 503.
If there is another error code, search for that resolution
Give Quickbooks time to resolve any server issues or reach out to them directly.