As you're coding you'll definitely do things differently. You can even learn from your competitors mistakes, because you can regard their product as your "prototype". And because you have a competitor you can be pretty sure there is a market and (potential) customers. This is a good thing.

There are always a million reasons to not go through with something. If you're excited about the idea, and if you're pretty sure you can turn it into something profitable there's no reason not to go ahead.