Lines in MA43

I am looking for documentation on the MA43 screens, and the process of adding line items. Can anyone point me towards a user guide on this process? I am trying to debug a process that is adding invoices via AP20, and line items via MA43. We are seeing a condition where the AP20 works fine, the MA43.1 works fine, but when we look at the AP20, the goods amount is twice the invoice amount, even if a single line item exists for the proper amount. When I drill around into the invoice lines, the line item appears to be there twice, which I think is related.

I am hoping to decipher this issue with some documentation. :)