Dewalt d51276 fires multiple times with a single pull of the sequential trigger

what do you mean fires two at one time or two like bam bam.? different things all together. two at one time is the nose piece and normally occurs on coil nailers when they wear out. If its bam bam then, yout trigger is black and its called a contact tip trigger, meaning as long as the safety is pressed down and the trigger is held it will fire. But common problem is when people use a gun and put it on the board and pull the trigger. Like any gun there is a " kick" the kick is the operation of air going through the gun hitting the piston which is attached to the driver, pushing the nail into the board, ( a lot of energy directed straight down) pushing the gun up. and the trigger while released actually is making you pull it twice. So, as a tech I teach new people that there is a sequential trigger that does not do that or if your good with your aim try holding the trigger and tapping the board a single time.
but long and short of it, nose worn ( pricey) or trigger

Could be one of several things - first, check the contact element (part that touches the wood) make sure it is not bent & moves up and down with the linkage freely.
These are also very sensitive to volt/amps - if your battery is not up to par you can also have malfunctions like this.
Last, open the front metal cover (3 screws) and inspect to see if the return bands are in place and not broken. These are located at the top on either side of the driver blade. This is the most common repair on this tool.
Let me know if this helps.

Remove the head and lube the headvalve with white grease. If gun still does not fire and makes air noise at the trigger valve then orings within the trigger valve are worn. Replace the trigger valve or if you have a source for small orings, just replace the two orings on the metal stem (2mm x 1.5mm). Dewalt uses vinyl orings on the stem that fail. Replace with buna. Good luck

We often see jamming with stainless wire collation - the problem is almost never the gun, it is because the stainless wire is much harder to shear off than regular steel. I would suggest trying another brand of nail and see if that helps.
Also, yes, the trigger is all that is required to change it to sequential, it is a very easy swap.

Most nailers have a two position setting on the trigger to allow for single fire or bump fire. If set on bump fire, the gun will work better when you bump fire the gun instead of placing the gun on the work then depress trigger. Bump fire works best when safety spring has good tension. Easy test. Make sure that the safety spring is in place (with air hose removed, contact foot should spring back quickly when pushed against work). With air connected, hold trigger and quickly bounce the gun against the work. If the gun does not fire when you bump, the trigger is most likely set to single fire. With trigger set to single fire, you must place the gun against the work, then depress trigger. If the gun shoots two nails while set to single fire, most likely worn trigger or worn out trigger valve. These parts are not expensive and are easy to replace. Good luck

Worn trigger valve orings will cause gun to fire twice, and /or attempting to single fire nail with bump-fire trigger. Easy test, look closely at firing pin under trigger. It should fit snuggly an not move from side to side in valve. If loose, replace valve. If your nailer has bump-fire trigger, do no attempt to place gun then fire one shot because the gun will most likely double fire. Good luck.

The link below is where you can download the owners manual. The link is on that page near the top right hand side under the area marked downloads. Page 5 will show you were the switch is located that switches the tool from sequential fire to bump fire.

DeWalt DC618 parts and operations manuals

Check this notice and post your question again if you nailer is not part of this recall. Good luck.

D51844 & D51845 Recall Notice Published 06/09/2005 08:49 AM ' Updated 09/29/2011 09:35 AM What is the recall on the D51844 and D51845 about? The contact trip on some of the nailers may not operate properly, which may permit a nail to be ejected with only trigger actuation.

Only nailers with the following date codes are affected:

200203, 200204, 200217, 200302, 200304, 200308, 200315, 200325, 200329, 200343, 200403, 200404, 200406, 200407, 200412, 200413, 200415, 200416, 200417

See the link listed below for illustrations & more information regarding the recall.

http://www.dewalt.com/us/articles/article.asp?ID=1373

Dewalt has two triggers available for their framing nailer. The black trigger (390746-00) is a bump style trigger. While holding the trigger, you will be able to fire a nail by pressing nose against work. This trigger is preferred by most framers because it is much faster. Gray trigger will only allow one nail to be fired. Bumping will not work with the gray trigger. With trigger released, you must place the nose against work then pull trigger to fire. To fire next round, you must release trigger, pull gun off the work, again place nose against work then press trigger. Check dewalt web site for recall information. Some dewalt framers have been recalled for faulty trigger. Good luck with your projects.

It sounds like you have a contact trip trigger that you are trying to operate as a sequential trip. Sequential trip is when you have to put the tool against the work and pull the trigger every time, contact trip is when you pull the trigger and tap the nailer on the work and it fires. If you operate a contact trip like a sequential it will fire but it usually fire more than one nail. Your instruction manual will explain this, sometimes they give you two seperate triggers for the two modes or there is an adjustment on the nailer.