I've been experiencing this too. Most commonly when I'm converting an unstructured reference to a template and have left some non-template text in the reference content.

Thanks! With your clue, I was able to figure out the exact steps to reproduce this issue:

1. Edit a reference with no template
2. Switch from "No template" to some template
3. Add any text to any parameter
4. Update the reference

This inserts duplicated wikitext. I'll try to fix it now.