Duplicated serial number in material documents

Dear Gurus,

As  now I'm facing with a problem which serial number X of material Y has been duplicated in 2 material documents.

First I created a matl. document with movement type 281 - GI for network, which issue 1 pc of material Y - this pc has serial number X.

After that I created the same matl. document with 281 - GI for network, which issue 2 pcs of material Y - however system picked 1 pc with serial number X and 1 pc with serial number X+1.

So serial number X has been duplicated in 2 material documents.

I found some SAP Notes, however they are all obsolete. Could you please help me to identify the problem and the solution to fix it?

Thanks