Click the gear icon and deselect "Use Frame Range".
This normally does the trick but can be buggy with bones animation.
What helps is to either restart MED (if you get lucky) or to press the "Auto Scene Range" icon in the lower left animation bar which then usually only messes up frames of the same scene. I just thought of this but you could try and rename every 2nd frame if you really have to, so the auto scene range only takes 1 frame long scenes into account.