All of a sudden, or rather we just noticed it, we can use the same serial number when we are finishing production on two different work orders for the same item #.
 
Not good!
 
These work orders were created by converting a SO line item for 2 to a work order, then entering multiple dates and splitting the one work order for 2 into two work orders for 1 each.  
The serial number used was finished and shipped last month.  It was accidentally entered on the second work order and it was accepted.