STEPS TO REPRODUCE
1. Navigate to Upload Rules from RosterCP
2. Click on checkbox for "Default"
3. Enter Guild Name into Guild Name field
4. Enter Server Name into Server Name field
5. Enter Region into Region field
6. Click Add
7. Click ctrl-shift refresh
8. In Firefox, you are provided with a POST message warning.
9. Click OK on this warning.

Note for every time you refresh, a new entry for the Guild created in steps 1-6 is created on the page. (Although the Guild table in the database doesn't appear to be updated when this happens.)

What version of PHP are you using? 5.1.6
What version of MySQL are you using? 5.0.27-standard-log
Is it your own server or hosted? If hosted, by whom? (we see heaps of limitations and problems with hosted services, eg. GeoCities, Yahoo, and others) BlueHost
What version of CharacterProfiler, GuildProfiler, and PvPLog (if being used) are you using? CP 2.1.1, GP 2.1.0
What language is your system, server, WoW server, etc.? English

This is the nature of post forms on the web

That error in FireFox is asking you if you want re re-submit the data that was sent to the page when it loaded.

You are telling FireFox to re-post the same data

Understood. Simply a usability issue of trivial severity. Especially when one considers it can only be encountered by those who have access to the RosterCP.

Nothing can be exploited with that at this time, as it just inserts more rows. I guess it's entirely possible that someone could refresh it millions upon millions of times, and fill the filesystem, but with each new row taking MAYBE 1kb, it would take quite a while.

I'd say this isn't serious enough to add checks for. Duplicating rules doesn't affect whether or not uploads are accepted.
Say you have two character rules: wrath% and %bringer. A character called wrathbringer will be uploaded only once, even though it meets both rules.