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Exactly the error I was seeing and much better than deleting the rows in the comment table which how I was initially solving the problem.

I did run into a little problem with figuring out where to put the process plugin. I eventually figured it out with the help of this article from Advomatic

The file system in Drupal 8 is organized very differently than in Drupal 7. Within your custom module, plugins will always go in [yourmoduledir]/src/Plugin. In this case, our migrate process plugin goes in [yourmodulename]/src/Plugin/migrate/process/[process-name].php.

Thanks again.