You can check this using Cmd-Shift-Y, then look into the “Plugins” folder. There you will probably find the several versions of RMX.
The problem is that Glyphs only overwrites existing versions of a plugin if it has the same name. In order to avoid this problem, you should not rename the RMX.glyphsFilter (although it would make perfect sense to do so).
As I suggested above, ideally, this problem should be addressed by Georg, not the maker of the plugin or the user. Glyphs should check for existing plugins of the same name (not just file name) on install – shouldn’t be too difficult to implement.