I have seen that other people have complained about this. When there are lots of tabs, Firefox becomes slow and, sometimes (more often than it should) doesn't load some tabs (they stay on about:blank). It usually happens when loading a session or after a restart (because Firefox does a session reloading). That triggers this add-on, which promptly clears all about:blank tabs, except one, as they all look the same. It's a problem with Firefox loading sessions, but made worse with this add-on's action, which doesn't wait for the tabs to fully load before cleaning (actually, I don't even know if that is possible, as it's a Firefox problem handling more than, say, some hundreds of tabs). A solution could be to not automatically close duplicated tabs, just new duplicated tabs, with an option to manually trigger a duplicated tabs clean-up.