A document with number XXXXXX already exists?

Hi to all,

My interface is file to idoc.

when i try to post then data to sap [ "A document with number XXXXXX already exists"].

The above error is come.

I am also change the reference no but same error is come.

my idoc status is 51.

why these type of errors are come?

It's data issue or XI issue?

can clarify it?

Thank You.