FULL PRODUCT VERSION :
1.8.0_45, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM

ADDITIONAL OS VERSION INFORMATION :
Mac OS X 10.10.2

EXTRA RELEVANT SYSTEM CONFIGURATION :
Memory Usage: 425 MB / 910 MB (287 MB allocated, but free)
Java version: 1.8.0_45, Oracle Corporation, Java HotSpot(TM) 64-Bit Server VM
VM arguments: [-Djava.library.path=/Applications/JOSM.app/Contents/MacOS, -DLibraryDirectory=/Users/xxx/Library, -DDocumentsDirectory=/Users/xxx/Documents, -DApplicationSupportDirectory=/Users/xxx/Library/Application Support, -DCachesDirectory=/Users/xxx/Library/Caches, -DSandboxEnabled=false, -Dapple.laf.useScreenMenuBar=true, -Dcom.apple.macos.use-file-dialog-packages=true, -Dcom.apple.macos.useScreenMenuBar=true, -Dcom.apple.mrj.application.apple.menu.about.name=JOSM, -Dcom.apple.smallTabs=true]

A DESCRIPTION OF THE PROBLEM :
This is an upstream bug report for:
https://josm.openstreetmap.de/ticket/11342

OSX users complain about all accelerators command+X (X being any key) firing actions twice.

Likely on a russian keyboard.

It seems like an old, non-reproduced problem:
https://bugs.openjdk.java.net/browse/JDK-7178856
https://bugs.openjdk.java.net/browse/JDK-7124217


REPRODUCIBILITY :
This bug can be reproduced often.