Okay, UPS has come & gone so we had to go ahead & print the packing list that has the same number as the one from last month.  Does anyone know what is going to happen when we post the 2nd invoice with the same number?

As far as I know there are two ways to get a Packing List number.  With SD-Q set to make the packing list and Inv# the same, this works when you release the sales order first (preassign an invoice number) and then print the packing list.  If you print the packing list before releasing, Evo assigns the next number according to what the last number is in the defaults.  
If you are changing the next number in defaults to be a invoice number, then the next time you print before releasing Evo will assign the next number(last invoice+1).  Now if you release a sales order and then print the packing list, that same packing list number may be assigned again.  Don't know this for sure, but a way to avoid it is to change the last number in defaults back and make that series very different from your invoice series.  Then at least you will know when the assignment is from the default list or from releasing the SO.
 
Also, I believe we have noticed duplicate numbers on packing lists  when  two lines are shipped in different shipments and therefore different releases, but we have not invoiced the first one.