I've been struggling with this one for a while, and finally realized that it's just an easy way of avoiding getting my hands dirty and a form of procrastination. There's really nothing bad that can come out of designing and building an app from start to finish, even if you're the only one whoever uses it. I finally dove into my first full app and if nothing else it's a great hobby and gives me something to look forward to after work.

That being said, I'm still of the thought that an identical clone isn't something worth sinking hundreds of hours into; I think you need at least SOMETHING that is different (if only just a bit). Example: After a recent 2 week cross country road trip, I created a quick and dirty travel log to track our adventure on our web comic site. When we got back, I planned to implement this as a web app, but discovered Everlater, which does EXACTLY what I wanted to write (even down to the exact target markets and usages..). Reading their development blog felt like they were stealing the thoughts out of my head, so when it gets to that point I think finding another idea is best.

Ideas are a dime a dozen - if you need help, just use this list http://www.sixmonthmba.com/2009/02/999ideas.html.