Technically, I suppose that doubling (or double-casting) isn’t a playwriting technique: it’s more a casting/directing technique. On the other hand, if your story requires a decent number of characters (especially bit parts), it can sure help to keep doubling in mind when you’re writing the play.
Doubling: when the same actor or actress plays multiple parts in the same performance
For the most part, doubling is used for background characters (waitresses, clerks, people walking by, etc.). Legally Blonde: The Musical is an extremely good example of this kind of doubling. If you watch closely, you can follow specific actresses and actors through a variety of roles (most background characters play at least three roles, and even the actress who plays Vivian has a smaller role before that character is introduced).
If you want a big character list on a tight budget, doubling can help. In fact, as far back as Shakespearian times (possibly earlier), small troupes have used doubling to perform plays with more characters than they had actors – or more actors than they could afford.
Can’t afford another actor? No problem! Have Bob play two parts.
Think about it: yes, you still need two costumes, but you only have to pay 1 actor. That makes your play easier to perform, which makes people more likely to perform it. As a playwright, all you have to do is to make sure that certain characters aren’t written into the same scenes. If two characters are onstage at the same time, those roles can’t be doubled (unless you have a really creative director… The Flying Karamazov Brothers did manage it).
In any case, doubling is an opportunity that’s easily overlooked when you’re writing (that’s the director’s problem, right?). You may not always want to use it. You may use it all the time – it’s hard to say.
But if doubling really works for the play, wouldn’t you rather think of it as you’re writing instead of having to go back to make it work later?