DMT showing "Error occurred reading the file: An item with the same key has already been added."

I’m trying DMT for the first time, and things were going smooth until I got the message below. I’m using a “PO Combined” file, and the first upload worked just fine. The second upload didn’t go as well, and it’s the message below showed up.

There were no changes made to the file before the first and the second files, so I’m not sure the cause of the error. Any suggestions?