I uploaded my class list twice. The first time it was in the wrong template. I fixed the problems and uploaded again thinking it would overwrite the first entries. Now there is two of everyone. How can I just delete the list and start over without having to delete individually?? Thanks.

Manage Classmates > Upload Classmates > Scroll half way down the page and click Reset Classmates. On the following page, scroll all the way to the bottom and click Delete All Checked Classmates.

Note - This action is permanent and cannot be undone.

If the same list gets uploaded twice will all of the classmates be added twice. (assuming columns were mapped correctly) or would the second load be treated as duplicates.
If so, what is the keyfield that the import from csv uses to determine duplicates?

The columns that are named and on our upload template that we provide, need to remain in their order, named as is, and no additional columns may be added. Any changes will cause duplication and other errors during the upload process. I would nuke all and start fresh. NOTE: this process is only intended for the first upload of classmates. If this process is carried out once members join, profile information will be lost.

I agree with what you said, but my question was more academic.
If the same list is uploaded two times, will the system accept the second upload and add them all to the database a second time or will it detect and reject them as duplicates.

If they are rejected as duplicates, what are the specific keyfield(s) used to detect this duplication.

It rejects them as duplicates if the first name and last name match.

thank you so much.