A couple of my friends started a nail art page on instagram a while back. I am pretty good at it and was thinking about starting one myself. Would this be stealing their idea? Would it annoy them?

Oooo good question! Well, I think it would kind of annoy them because you would be copying their idea. And you would be competing for followers and stuff...

On the other hand, everyone is inspired by someone else. I doubt their idea was 100% original, maybe they saw something on Instagram that have them the idea in the first place!

My advice would be rather than going for exactly the same thing, why not try something slightly different... Like you could do it on toenail art and call it insTOEgram or something :)

That might even be a bit too similar though, so you could branch out to makeup tutorials maybe, or pictures of cool hairstyles, hmm ... Whatever you're into really.

Then people who show an interest in your stuff, you can recommend they check out your friend's page too.

You could also try out Pinterest or Tumblr or other sites instead of Instagram, that would make you even more unique.