I put together this example file. (Google Drive treats capx files strangely. Just click the "download" button in the top right.)
To keep the player from running after the first tap, I put the conditions for the first and second taps as subevents of an "on D pressed" condition, separated with an "else."
I also noticed 0.05 seconds wasn't really enough time to double-tap. I barely managed to do it with the timer set to 0.25!