well, the user could always try to send replicate information, there are ways to prevent this. It all has to do with what your program does, and how you set up your mysql tables. You can specify a table column to have unique entries, commonly used for id numbers on entries, you could simply perform a query using the same user info, to check if they had submitted this information before, if a record is found, it does not write the information in the table, much like this site does if you try to submit a question with the same title. There are several ways of performing this task.
