Does this happen all the time? Sometimes?

When the browser looses contact with the site, or other issues, the browser will ask if you want to send the information (that had been sent) again,

Many site issues can be caused by corrupt cookies or cache.

• Clear the Cache and

• Remove Cookies
  Warning ! ! This will log you out of sites you're logged in to.

Type about:preferences<Enter> in the address bar.

• Cookies; Select Privacy. Under History, select Firefox will Use Custom Settings. Press the button on the right side called Show Cookies. Use the search bar to look for the site. Note; There may be more than one entry. Remove All of them.
• Cache; Select Advanced > Network. Across from Cached Web Content, Press Clear Now.

If there is still a problem, Start Firefox in Safe Mode {web link} While you are in safe mode;

Type about:preferences#advanced<Enter> in the address bar.

Under Advanced, Select General. Look for and turn off Use Hardware Acceleration.

Poke around safe web sites. Are there any problems?

Then restart.

I think that prompt would only appear if Firefox were submitting the form to be website twice. I'm not sure whether this is a feature of how login works on that particular site, or whether the form is just really sensitive. When you click the submit button on that page, be extra careful to only single-click and not double-click the button in case it submits twice on a double-click.