A few more details. I am updating a 550 MB DB, which upon further review, was perhaps causing php to timeout during the upgrade. I did find in the error logs the message "Maximum execution time of 300 seconds exceeded". During the upgrade, after the error message, I would have to reload the web browser, which perhaps was executing the same code and causing the table already exists DB error.
I used this stackoverflow post to assist in resetting the max_execution_time to no limit.
In my last sandbox attempt to upgrade, after making the environment changes to max_execution_time, the DB did not time out and produce any error in the log. The upgrade took about 25 minutes.