While releasing a SO (using SO-E) with a qty 6 of a single serial number controlled item,
the option to do serial control was selected.
 
The standard screen for specifying Serial numbers displayed, but any serial number
entered was rejected with a error that the serial number already existed.
 
This would be true, because all 6 items were turned in as Finished Production and  
the serial numbers were created at that time.  We just wanted to specify which
ones should go to this order.
 
We had to ALT-CTRL-DEL to get out of this SN assignment mode.
 
I then went into SO-A, brought up the SO, Saved it and selected yes to
the serial control question.  
 
From here, (the screen looked the same as the one in SO-E)
I could put in serial numbers and it accepted them fine.