Error when posting in the sales invoice

when i am trying to post from the sales invoice the following error occur:

“Cust. Ledger Entry Entry No ‘X’ Already exists”.

Any idea ??

Did you/any of your colleagues Add/modify/delete Cust. Ledger Entry Entry table?

You mean manually in the cust. ledger entry table?

Yes, how to resolve this

What exactly did you change? Is there any modification in posting routine (codeunit)?