How do you make sure you don't accidentally give all the punchlines to the same character who I will nickname Joe Surrogate?
If this is happening in your script, it's probably not an accident.
Best case scenario: One character having all the jokes is an indication that you don't have a sitcom yet. Just one character. And that's fine. It's a start, you can build other characters and conflicts around that character.
But it's also asking why your character has all the jokes - and what sort of jokes they are. If they're normally jokes about nothing in particular, rather than character, you've got a bigger problem. In which case, you don't even have a character. You just have someone who pays off jokes.
In Friends, the writers could easily have fallen into this trap with Chandler, because his character's 'thing' initially was 'being funny'. His jokes were the sum total of hours of thought from some of the funniest minds in America. Funny guy. But what the show brought out over time, and what is actually funnier, is that Chandler is coward. Fortunately, they could take their time over this because there were five other brilliant characters who paid off each others' jokes.
Another show that leaps to mind in this case is Shane, a perfectly watchable vehicle for Frank Skinner that didn't do very well when it came out in 2004, despite Frank Skinner being one of the funniest, sharpest guys around. When I saw it, I thinking it was perfectly fine, but also remember feeling that the main character had all the punchlines, and that everyone else's reason for being was purely to lob up set-ups so Frank could smash them. And having just watched a bit on Youtube just now, some characters have some lines that no-one would ever say, but it has to be phrased that way for the joke to work.
It's the exact opposite of what Jerry Seinfeld did in his sitcom vehicle, in which he plays a successful comedian. But he isn't actually the funniest or most interesting character. In fact, he's probably the least funny and interesting - and the worst actor. But we had other characters and conflicts that made the show work.
It may be you have a bunch of characters who aren't quite gelling or fizzing. In which case, there are a couple of things you could do.
Try pushing them to extremes
Let's say one of your characters is a pretty straight and earnest. In which case, make them ludicrously, painfully, infuriatingly straight. And honest. And earnest. And innocent. That will generate jokes for them - and at them.
Try putting your characters on the front foot
So, they become the kind of characters who want everyone to be like them - whatever it is. Let's say another character is fastidious and tidy, and the butt of jokes. Try giving them some attitude and insisting that everyone else be as tidy and uptight as them. That might produce conflict, stories and therefore jokes.
This kind of stuff takes ages, even if you're experienced and doing it full time, so don't get disheartened if it's not happening at the moment. Your sitcom episode is essentially a recipe that can be repeated again and again, producing roughly the same consistent enjoyable result every time. Recipes take a while to perfect, but one you've got them right, they really come into their own.
Hope that helps. Thanks for your question, @MagsTheObscure. Keep 'em coming.