"the same info" is pretty generic. Depending on the actual data you're talking about, it can be either easy or hard.
<b>Easy</b>
Say you're creating some kind of website that needs to have "user accounts", and you've decided to use email addresses as the user's visible ID* since they are guaranteed to be unique. This is easy, just make the column that stores the email address UNIQUE, and the dbms itself will prevent any duplicates. You will want call the query function prefaced with <b>@</b> to turn off PHP's automatic error reporting, and instead look at the error result to see if the failure is because of a unique-key violation, and respond appropriately.
<b>hard</b>
You're creating some kind of online poll or election, and the duplicate information you want to prevent is the possibility of the same person voting twice. I won't bother rehashing the arguments here--lots and lots of electrons have been spilt on this site and others on this topic--other than to wish you a (not at all optimistic) good luck!
