Porter Cable Finishing nailer FN250B

Page 8 of the user manual (found with a simple internet search) indicates that the nailer has a "Contact Actuation Trigger", commonly called a "bump trigger" or "bump fire mode". As long as the trigger is held down and the nose is in contact with the wood, it will fire. The bump trigger is black and the single fire trigger is red:

1. [SINGLE SEQUENTIAL ACTUATION TRIGGER]: To fire, grip tool firmly to maintain control, position nose of tool on to work surface, depress safety, and squeeze trigger to fire a fastener. Allow tool to recoil away from work surface as fastener is driven. This "single sequential actuation" method provides the most accurate fastener placement.

-OR-

2. [CONTACT ACTUATION TRIGGER]: To fire, grip tool firmly to maintain control, squeeze and hold trigger, push the tool firmly against work surface. Allow tool to recoil away from work surface as fastener is driven. If safety element is allowed to recontact work surface before trigger is released an unwanted fastener will be fired. The tool will fire a fastener each time the safety is depressed. This method is known as "contact actuation" and allows very fast repetitive fastener placement.

IMPORTANT: The "contact actuation" trigger is available free-of-charge by calling 1-800-321-9443 in the United States and Canada or 001-731-660-9374 outside the United States and Canada, andp roviding tool model and serial number. For identification purposes: the contact actuation trigger is black and the single sequential actuation trigger is red.

^{Image and text from the Porter Cable instruction manual for the FN250B finish nailer, hosted at the link above. Emphasis added.}

Is it possible that you've got this option installed (I recently purchased a different brand of finish nailer and mine came with the bump trigger & I installed it) and are simply moving a bit too slowly? If you squeeze the trigger then don't release the trigger immediately or allow the recoil to push the nailer away from the surface it will fire again because that's what it's supposed to do.

On a test piece, try squeezing the trigger, then quickly pushing the nailer against the wood and immediately pulling it away. If you get only 1 nail fired, this is the answer. You'll just need some practice to use the bump trigger, or replace it with the single-shot trigger.