I know that this may be a little late, but the solution I use to only allowing a user to upload one score and have it update their previous score if they get a new one (not using the name identification method, since more than one user might enter the same name) may help someone else out here. It requires a little bit more knowledge on php and how database tables work though, but there are plenty of php and SQL tutorials out there, so if you search, you should be able to find one!
In the game's .ini file, have a "playerID" variable. When they go to submit a high score, have the game check that variable. If it doesn't have a value, that means this user hasn't submitted a high score before. In this case (this is the part where php scripting knowledge comes in handy), have the script pull up the online high scores table sorted by the playerID (you'll have to add this column when creating the database), get the highest playerID and add 1 to it, and write the player's score to a new line in the table, including that new incremented playerID number. Remember to also write that new playerID number to the local .ini file, so that it can be referenced in the future! Then you'll re-sort the table by scores (highest to lowest) and show the top scores. Since the playerID is generated on the server and incremented each time someone new submits a high score, each player will have a unique playerID!
When someone with an existing playerID (written in their .ini file) submits a high score, you'll just tell the php script to locate the entry with that unique playerID, and overwrite those scores (if they're better). Voilà!
I know it's easier said than done, and I wish I had time to write a tutorial about the php side of things, but again, a quick search on the internet for tutorials returns a plethora of resources. (It's how I got to where I am today!)
I hope this helps someone out! 
