Are we allowed to replay the levels in GC? I'm unable to get past a certain level, so can I replay the ones before it?

yes you can .. esp. that u need 50 stars to get to the next stage .. im currently at 42 and replaying some levels

I've made it to the second stage, I just can't get past level 23.

I think we can, even if we've passed the second stage. I know I have just so I stop feeling so demoralised by not passing whatever level I'm stuck on!

Yeah you can replay levels :) Thats the only way to move to the next stage unless you were able to get 2-3 stars on all of them the first try. Though replaying the levels doesn't give you a free power up to keep for later, like it does during the first play through.

But it does give you the NP again (= Even if you re-play the same levels over again. I've done it with levels one and two.

You're allowed to replay any of the levels as many times as you like! I'm already in the second area and I still can replay levels in the first area. Besides aiming for full three stars for levels that you didn't get three stars on the first try, this is also a way to make some quick NP, as mentioned above by Dawn. For instance, just replay level one or two 10 times with three stars (highly impossible to get less than three stars lol) and you've got yourself an easy 10,000 NP. \o/ Hope TNT doesn't take away this feature though. That'll be sad.

I do think that you can receive NP from one level only so many times. I stopped getting the NP message after a while on level one.

You can indeed replay levels as many times as you like, but you're limited to receiving exactly 50k NP a day (I've been observing my earnings carefully over the past few days). ^_^ I've just been replaying easier levels for the Neopoints, hehe.