I figured it out!
I was linking a template box & a linked datacard box to the serial number so it was assigning the serial number twice.
I changed the "Default value" in the properties of the datacard box from "serial Number" to "Text Value" while leaving the same variable name.
I had the same problem. Similar cause, but what I found was...
- I had created a edit box on the template card to display the next serial number.
- I copied the edit box and pasted into the data card (lazy... didn't want to make up a whole new edit box).
Problem was because I copied from the template, both edit boxes were pulling their "default value" from the serial number generator.
I changed the data card edit box to pull the default value from "text value" (and made it read-only) and everything worked fine from then on.