For inputs: it's known problem. Try to disable IBUS or similar u have enabled by default and double input will go away.
I know. But I also need to warn the end user and forcing change the configuration of your system
What GTK_IM_MODULE env variable says in your case ? Maybe I can add it to the blacklist.
See http://bugs.freepascal.org/view.php?id=26926
Ok. It's xim, recently added to the blacklist list yesterday, I must try again. Thank You. BTW is not just lazarus, reading online seems that another software is affected also, like Inkscape.
The most problematic thing is that all the non working modules comes all as default in ubuntu and their derivative distros xim, scim and ibus and is not as easy for new linux users to disable it.