I have a simple scene with a UI InputField. When I run (via the editor), I get duplicates of very character I type, e.g. I type “this is a test” and the field gets “tthhiiss aa tteesstt”
I’ve tried the same project/scene on MacOS and is works correctly in the editor and when I build a stand-alone application.
I’ve tested this program (built executables for) on Linux, MacOS and Android. It runs correctly on MacOS and Android, but on Linux I can never type into the input field 
Sounds like Unity is taking input from to source at once. Try unsetting GTK_IM_MODULE before launching Unity, like so:
unset GTK_IM_MODULE
/opt/Unity/Editor/Unity
Btw, what distro do you run, and how did you install Unity? If you’re using Unity from the AUR then the launch script already does that for you, and the problem may be laying there.
I don’t have that set in my environment. I also get the same results either running Unity from the command line (as above) or when launched by KDE.
I’m running KUbuntu 14.04 (basically Ubuntu 14.04 with KDE desktop preinstalled)
To be clear, I installed using the automatic packaging from unity-editor-5.1.0f3+2015090301_amd64.deb
Seems to be fixed in the 2015-09-15 update