Ask HN: Is it bad to entertain an idea that has already been tried?

I'm playing with an idea in my head that would be very fun to implement, but the idea is not particularly original and although I haven't checked out how many sites have done the same thing already, it feels a lot like the idea is not worth implementing except for fun.

So, I'd like to ask you whether it's a waste of time to entertain ideas that have already been tried (successfully or not)?