Issue

Posted Invoice(s) exists for an unposted Invoice(s) the user is trying to delete.  Message = "Invoice XXX already Exists"

This situation can happen if the No. Series is updated and Invoice No. used twice on 2 separate invoices.  The cause of the message is because another Invoice was posted with the same Header No.  When NAV deletes an invoice, it wants to post a blank invoice with the same Header No. to eliminate a gap in the number series.  But NAV could not post this empty invoice because previous invoice was already posted.

The solution is to take off the Posted Purchase Invoice No. Series for the invoice(s) in Object Designer, Table #38 Purchase Header.  This will allow the invoice to be deleted without automatically posting a blank invoice.  Normally this isn't recommended but because a previous invoice was posted no gap will exist in the No. Series existed.
Solution

-Confirm posted invoice(s) exist and duplicates are ready to delete
-Object Designer #38 Purchase Header > Run 
-Remove Posting No. Series for Invoice(s)
-Close Object Designer
-Delete Invoice(s)