Can I double excess heat tape on a pex pipe?
I would not double back. I would consult your heat tape/cable directions. If you no longer have the directions, I would google the make/model of your heat tape/cable to see what the manufacturer recommends when it is too long for what you need. 

In general, if it has a plastic piece at the end (as the one I used did), I would not cut it. If you can't cut it, just put it a little longer and use a small electrical cord staple to affix it to the joist beyond the pipe. 

It should be perfectly safe to put the tape/cable next to wood, most of these heat tapes/cable are designed to keep the temperature just above freezing, and are self-regulating to prevent them from getting too hot (as long as you don't cut them when they shouldn't be cut :-). I would definitely do a little research to get the manufacturer recommendations.