When creating a character, if you choose a name that's already taken, a message will pop-up saying “This name is already in use.”

Quote Originally Posted by Magnesium02 View Post
There will be guidelines that stop people from stealing oneone elses name though right?
In the event that someone has the same exact first name and last name after we transfer characters from FFXIV 1.0 into the worlds used in ARR, we will check who originally had the name determined by certain priorities. As for what those priorities are, we will be announcing this on Lodestone in the future.