Error in Idoc is 51

I have posted one Idoc and it was succesfully generated.

But I don't want duplicate Idocs ( with same data ) using we19.I was using function module to avoind duplicates.

But I have generated next Idoc...

Have got

Error in Idoc is 51.

Check if invoice already entered under accounting doc. no. 1600002035 20

Is it okay.. is it considered as stoping of duplicate Idoc generation.

Thank you.