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This is due to a formatting issue in Excel.
In order to resolve this issue you should save the .xls file into a .csv file (comma separated file). View this file in notepad and you should notice extra commas added at the end of the list of your columns. Remove these extra commas and reopen the file in Excel and save it as a .xls file.
At this stage you should go back to the original .t file and go though the data driven process again, creating a new .g.t file during which you will reconnect to the modified .xls file. The extra columns should no longer be in your column list.