However, as of sometime in Spring 2015, we currently have to individually ignore each of those summary columns at the end of the CSV file (the assignment group summaries, the Current/Final score, and the Current/Final grade) by marking them them as "Bogus, ignore" or remember to delete them out of the CSV before upload.
Read-only columns as part of a Gradebook comma separated values (CSV) file are not considered new assignments.
Explanation: When a grading scheme was enabled and the Gradebook exported for a course, the Gradebook created extra read-only columns. When the Gradebook CSV file was imported with no changes, Canvas considered the extra columns to be new assignments. This behavior occurred because the importer was not checking the status of the columns and did not see that they hadn’t changed. Canvas code has been updated to disregard any columns or assignments that do not include any changes from the original file. This read-only column change also applies to assignments that do not include a grading scheme.