Hi,
As mentioned above use NAST table or a ZTABLE to compare if its the first print or not and store the names of original and duplicate watermark into a variable using the IF-ELSE condition.
Then you can pass the variable name to the background picture of the smartform as shown below.
check the below example for printing a watermark using dynamic values fetched into the variable.
You might face error while checking or activation stating to enter a valid variable or image name. In that case it is important that you pay attention on the arrow at the end of each field of Name, Object and ID. The arrow should be marked to the left as shown in the picture to select a dynamic variable