Nailer will get two nails jammfrontiered at same time about every 3rd nail

Sounds like you may have a nail jam. You didn't specify what type of nailer you have - most finish guns have a latch to open the nose & clear a jam - if it is a framer most of the time it requires a long punch and a hammer to push the driver back in past the jam to clear it.
Let me know if this does not fix your issue.

Nailers that use strips of nails (one nail directly behind the other) will sometimes drive two nails at once and is caused by damage to the driver. The nails are so close together that the driver will hit the first and part of the second nail head. Coil nailers, as your an451, use nails seperated by welded wire making it very difficult the the driver to hit the second nail. Usually when a coil nailer shoots two nails, it is acutally the trigger valve that acutates twice (two very rapid fires). Two very rapid fires is caused by faulty trigger valve. I have seen a roofing coil nailer fire one nail and a second nail drops out of the gun. Caused by weak / malfunction of nail feeder or wire used to weld nails together is too thick.

There are two issues that people have with nailers that sound the same however are totally different. Firing more that one nail at a time can be taken as; two nails being driven with one fire or the nailer vary rapidly firing a second time almost instantly after firing the first nail and each time only drive one nail. Problem of double fire (either type) is very common with nailers (the gun very quickly fires twice), however two nails at once can only happen if the wrong gauge nails are used in the gun. A 15 g nailer can very easily pass two 18 g nails or, with more difficulty, pass two 16g nails. To help prevent the driver blade from hitting two nail heads at once, most drivers are slightly tapered at the tip so that the driver will hit the first nail and the tapered part will push the other nails back into the rails slightly. Remove the head of the nailer and pull out the driver. Measure or compare the thickness of the driver to the thickness of the nail head. If you are sure that you are using 15 gauge nails, check the driver for taper. The tapered part should face the rails. If not tapered, try carefully grinding a taper keeping the tip cool so that you do not effect the hardness. You could also check for other brands of 15 g nails with slightly more spacing between the nails. There is a big difference in quality among brands of nails. Good luck with your repair

If it is a nailing hammer for a roofing project, I have used a degreaser to spray the end. Even on my air nailers that I use 16d nails in I do this. Make sure you keep 3 drops of the (FOR THE NAILER) IN IT'S RECEIVING END EVERY DAY! This will keep the inside good and slick.

This is a fantastic nailer, very powerfull. Information about your nailer can be found below.

http://www.bostitch.com/default.asp?CATEGORY=BOS_FRAM_NAILER&TYPE=PRODUCT&PARTNUMBER=F21PL&SDesc=21%26%23176%3B+Industrial+Framing+Nailer+System

You should not need to lube the rails for the nails to feed. Your nailer uses plastic collated nails and should travel very easily inside the rails. I would remove all the nails and check for broken pieces of plastic from nail strips. Feeder should feel snug within rails and not wobble and spring should pull tight. When you install the nails do not release feeder against the nails with force as this could break the plastic strips and cause jam. Nail not feeding is usually caused by wrong nails, weak pusher spring or pusher guides broken /missing. Simple test for proper nails.
Place a strip of nails on the outside of the rails aligned with the edge of the rails and see if the first nail is perfectly aligned with the path of the driver. If the head of the first nail is angled back slightly from the path of the driver, the nails will not feed properly. Good luck.

look to see that you are shooting the right sized nails in it. 16 and 18 gauge nails dont interchange and the can get hung up under the plunger if the try to shoot at all

You have to oil the nailer with pneumatic oil, or you're using the wrong size nail

I worked at at tool rental center while attending college and learned from the old tool masters about nails and nailers. Rental stores have many different brands of nailers to rent and customers would bring their own nails of all brands. You will notice on the case/box that your nails will /should work on many other brands however the degree or angle of the nails is not always exacly as stated on the box. You should check with protractor the exact angle of your nails and also check the exact angle of the nailer. Use straight edge on travel path of the piston and another on guide rails to check angle of the gun. You will discover that all is not as stated as in a perfect world. If the angle is off slightly, the head of the second nail will get into the path of the driver. This happens mostly with clip head because the nail heads are so close together. The full round head is spaced further apart with less jamming. If angle is off slightly, take a look at other brands of nails with protractor in hand, the angle will be different slightly or choose a slightly thicker nail. Look for the nails to fit with bottom of nail hitting the barrel first. This will move the head of the nail slightly away from the center line. Finally do not release pusher with force as this will jamb nails closer together. Good Luck!?

This nailers are very inclusive , i have experience issues when i try other nails brands, and when i put DeWalt nails they work just fine. keep in mind that most nailers have a safety that when a couple of nails are in the magazine it wont shoot,

wrong gauge nails or wrong head type nails check that the magazine is not loose