Like others said, a clone can validate your market audience, but you would have already anticipated the growth path hopefully. Being first to market will give you a buffer and when you add features, your cloners will have to play catch-up. Trust (us). You're first online solution wont stay the same (otherwise it will fail anyway). Never forget to take customer feedback and tweak things when they say they'll pay. Quick - who is Twitter's Clone? Um... How about Linked-In? MySpace? Sure Facebook, but after how long and how many millions invested?
On the copywrite/IP issue, you have a couple options. Let's say you were the original Twitter and they cloned you. No problem: keep on eye on them, and once you realize they're getting funded - file a lawsuit (making sure you've got all the original IP stuff dated to back you up). The USPTO.gov site has some hints on IP protection. You can patent website solutions btw - and file to register copyrights. If you failed to execute and best your clones in the market, at least you can settle out of court for some nice change. Note, this might only happen before the clone is profitable and can hire enough lawyers to bash you. And it only makes sense to pay you off to go away (and stop scaring off their potential investors). Are copyrights enforceable? That's about the only way I know. Chasing clones and begging them to stop just wastes your time, fattens your lawyers and makes you look petty.
Other countries? So what? There are less than 10 huge markets worldwide. Worry about making it in 1. The money to be made on the WWW does not truly come from WW.