Way to keep Serial Number in Sink

Hi Gurus,

I am working with Serial number profile with process given below

1. There is one system which is non SAP where we are creating stock with Serial Numbers.

2. Through I-Doc all the data is coming to SAP.

3. we are creating material document with reference with data coming through I-Doc. (same serial no is used which is created with non SAP System).

4. But the problem i am facing to keep both system in Sink with serial numbers. I found one problem that user made GR manually in SAP with serial no and while I-Doc run system give me error that serial no profile is already created.

Can anybody tell me various ways or steps to keep both systems in Sink.

Thanks in Advance.

Regards,

Chatianya Parchure