a topic suggestion I'd like to see on your blog: When you're at a show and a comic before you hits a topic that you plan to cover, or uses a key phrase that you're planning to use-- do you bail on the joke or do you forge ahead? Should you point out the previous reference before your bit? How easily does one alter his set at the last minute?
How I handle it when a comic hits a topic I planned to cover: If they're making a similar point or taking the same angle that I'm taking, I usually bail on the bit. You'll just look like a chump if you repeat the same joke that someone else just did.
But if it's just a similar topic but a different take on it, I like that. In fact, I'll often riff off what the previous comic said as the opening to my set and then work into the bit I had planned. If you make it seem conversational, this is a great way to seem spontaneous and in the moment, even though it's something you've prepared. It's a little bit of trickery but it's a good way to build rapport with an audience.
When you sell a funny prepared bit as something that just came to you off the cuff, people usually dig it even more. Some of my favorite comics to watch do a great job of weaving between riffs and prepared material in a way that you can't even tell which is which. It's a whole artform in itself.
If the similar topic thing is really a big worry, ya might want to think about what you're choosing to talk about onstage. Topical stuff can be fun...but the more unique and personal your p.o.v. is, the less you have to worry about having bits that overlap with other comics.