Account Engagement Sync Error: 'Uniqueness error with http://pi.pardot.com/prospect/read/id/XXXX. Consider merging'

We recommend merging the records or trying to resync the record again. If the prospect records are synced to Salesforce, we recommend merging the corresponding Salesforce records and allowing the merge to sync down into Account Engagement. If one or both of the duplicate records are in Account Engagement, you can merge them directly in Account Engagement. If the issue persists, ask your Salesforce Administrator to review the deduplication rules in case there's any error there.

Then, when the error is fixed, follow the steps outlined here if there are several prospects with this sync error.