Hi Kent!
Unfortunately, once a GST return is recorded, it is not reversible.
Is the second return a part of the next quarter (or reporting period)? If so, keep a copy of the second return, and include it when you do your submission for the next quarter. You will have 2 returns in the system that you will need to add together to get the full amount you would submit to CRA.
The only other option would be to restore a backup, if that backup was done before the GST return, and won't cause you to lose too much data. But, if your backup is even a couple of days old, I wouldn't recommend doing this, as any data you had entered since the last backup would be lost.
I hope this helps!
