Rated 5 out of 5 stars

(Thanks for the ctrl+drag tip! This may become unnecessary if I can retrain myself.)

To make Duplicate install in Firefox 3.5:
- download the .xpi, rename it as .zip
- edit the install.rdf file so is something like 3.6.
- Replace the install.rdf file with your edited copy
- rename the .zip file back to .xpi
- drag the .xpi into an open Firefox window.

It worked for me in both Linux and Windows.