Hello! 
I am using a touch screen on Windows 10 and I noticed that simple button triggers twice when using “On Clicked” or “On Pressed”.
There a couple of forum posts addressing this issue, but all of the solutions feel like a bad workaround or introduce other issues.
I noticed the double trigger only happens when doing a “quick” button tap. When holding it for a short moment before releasing, it only triggers once.
It looks like UE is recognizing a click and a tap at the same time, which does not seem to be the case on long press. (At leas from what I can tell)
Mouse for Touch is disabled in the Project Settings btw.
There is another suggestion to set “bTouchFallbackToMouse=false” in the DefaultEngine.INI which created other issues…
Hope to find someone who really solved this problem, which seems to be present since UE4…