Serials Repeating in Two Orders

Hi,

We have a scenario :

One sales order X created containing Item A serials 123, 456 and 789.

This Order is ship Confirm.

After Ship Confirm of Order X, another Order Y is created with Item A and serials 123, 111, 222.

The problem is that serial 123 is repeated in orders X and Y, while it should never happen.

Any help about why it happened and how to avoid is highly appreciated.

Regards.