This is a bug on Google's side that is affecting a portion of our users. If they don't fix it soon, we will explore a workaround on our side.

For now, the issue can be resolved by removing all tasks and folders/tasklists from your Google account and uploading a clean copy from Ultimate To-Do List.  I've included step-by-step instructions below. A potential issue with this procedure is that you may lose old completed tasks. This depends on your setting for completed task retention in the Miscellaneous section of the settings.

Here's what to do:

1. Go to the backup and restore area of the settings and make a backup for safety.
2. In the account management area of the settings, select the account and choose the option to unlink from Google. This will stop synchronization and keep a copy of your tasks stored on your phone.
3. Log into Google from a web browser and delete all of your tasks and folders.  Google requires that at least one folder remain in place. If you are unable to delete a folder, just delete all tasks inside of it.
4. In Ultimate To-Do List, go back to the account management area and choose the option to link to Google.
5. After you've established the link, all of your folders and tasks should upload into your Google account and the duplication should not occur again.

I am having the same problem. Single task gets duplicated 1 to 3 times. I keep deleting individually. But this problem happens multiple times a day. Please find a fix.