Rated 4 out of 5 stars

The double paste with middle mouse button seems to happen because one paste comes from Autocopy (with Paste On Middle Click option) and the other from Firefox's built-in middlemouse.paste set to true.
Fix:
1. go to URL about:config
2. type middlemouse.paste in Filter field
3. double-click to set to false

Then the Autocopy option Paste on Middle Click works as expected.

This review is for a previous version of the add-on (0.9.7).