As lloydbudd says that fatal error indicates that the importer plugin has been loaded twice. This used to occur as a bug if the script died during the plugin activation hook, but after the plugin had been added to the active_plugins option. Firstly this was fixed in 3.0.2/3.1 and second the importer doesn't do anything on the activation hook.
Currently the only other way I can think of this happening if there is another plugin hooked into activated_plugin (or a similar action) which is causing the script to end after the importer has been added to the active_plugins array.
You could try deactivating all other plugins and then activate the importer to see if there's a difference. Or manual activation by adding to the active_plugins option and checking for a fatal error (as this one requires database manipulation of a serialized field I wouldn't recommend trying it on a production site or if you're not confident enough).