YC is a numbers game, most of them are going to fail or fail to matter and it won't be because there was 2 or more doing similar things. There's also room for multiple winners in almost every industry as well on the off chance that happens.

There are already YC startups competing: Exec and Pathjoy are doing house cleaning. Grid and Skysheet for online spreadsheets. Lots of ways to do photosharing. Heroku, AppHarbor and DotCloud as PaaS's, etcetera.

There's a very, very long list of companies here that have lots more examples:

http://ycuniverse.com/ycombinator-companies.php

but... what about the component of fear described in this article? http://paulgraham.com/growth.html

YC is at the beginning or early stage of most startups, acquisition is usually years later and there's a million things that stop a startup reaching that point.

Just worry about your startup not someone else's.

thank you. In this case, maybe to compete an existing startup could be even better :))