Can improper lubrication cause a Garand to double fire?

I agree- probably either worn parts, or more likely he simply did something wrong reassembling the rifle.

Lubrication shouldn't cause a gun to act like that.

No - the answer is not CLP or Grease. I have 3x Garands and never had any problem with CLP. I think you nailed it with soft primers.

It is called a slam fire.

Slam fires is that the firing pin floats and is not restrained from impacting the primer when the bolt slams forward. If the bolt slams hard enough and the primer is not a milspec primer (or CCI #34 for reloads) the firing pin could set off the round.

Wish you had pics of the primers after the incidents. You may find that at least one primer out of each incident is barely nicked - but enough to set it off. Fulton Armory link below has a great pic of this.

Change the type of ammo that you are using, the primers in your current ammo are probably too sensitive. Is your buddy reloading?

Out of the manual:

"The specifications for standard military ammunition include harder primers to withstand the slight indentation from the firing pin when the bolt chambers a cartridge. This slight indentation is normal. The use of civilian ammunition with more sensitive primers or handloads with commercial primers and/or improperly seated primers increase the risk of primer detonation when the bolt slams forward. This unexpected "slam fire" can occur even if the trigger is not being pulled and if the safety is on. Use of military specification ammunition will help avoid this."

Update 1: While it still sounds like a slam-fire, I did find this on a gunboard -

It's the secondary or primary sear , it is either worn or someone has attempted a trigger job . After you pull the trigger releasing the primary sear the gun goes bang then the bolt cocks the hammer and the hammer is caught by the secondary sear then when you release the trigger it releases the secondary sear and it is caught by the primary sear . But when you release it slow or "just right" it will miss the primary and we go bang again " you can probably get it to miss the primary dry firing it then cocking it and releasing it slow".

This is a little confusing as you stated that it has not happened since. Do the test procedure mentioned with dry fire just in case. I assume that when he cleaned it after the incidents, he did not mess with the trigger group?

I have a Garand Tanker from Fulton Armory and I use CLP on it and not grease. It works fine and has no issue with "double firing."

Now the thing about CMP is that it grades the rifles accordingly. Your friend may have gotten a really well used rifle and you just need to get a new hammer and spring. You can get that from Fulton Armory as well. It's difficult to say exactly what part is worn but likely just need a new hammer and spring.

Your friend may also not be used to a good trigger and unintentionally bump firing it. If the garand had a National Match upgrade, then that is likely the culprit and he just needs to know how to pull the trigger right.

My advice is to get a new hammer to see if that solves the issue. You'll likely need a new hammer spring and/or trigger depending on how worn out it is. You can buy the entire assembly off Fulton Armory for about 300 bucks if you just want to replace the entire thing.

Nope. Not lubrication.

Soft primers? Maybe. This is very easy to check for. You simply load a clip and only fire every other round and eject the unfired ones after they are chambered. If the primers are 'soft' you will see dents in them from when the firing pin and bolt went into battery. No dents = no soft primers.

The worn part you are looking for that is causing this are called the secondary sear and it's release. It's the part responsible for holding the hammer back when the bolt is cycling and going into battery to prevent firing out of battery. Either this part is bad, or, the trigger is. Both are cheap parts available online. If you don't want to do the work yourself, then buy higher quality parts and drop them and the trigger group off at a gunsmith.

I'll agree with those who say it isn't a lube issue.....military type rifles are built just a little better than that. If a simple thing like CLP causes problems....the military would never have accepted them into service. The M-16 originally had a lot of problems in a jungle environment and a few design issues that never fully revealed themselves under the test criteria of that time. BUT...when the grunts started to use and abuse them, the lack of a chromed gas system / barrel, became apparent. Never an issue after that and the rifle is still in service.

If using CLP was an issue, the Internet would be full of people listing the same problem and the CMP would likely warn people to use proper lubrication.

My guesses are improper assembly the first time and proper assembly the second time, or that he was bump-firing it until he got used to the trigger and started holding the rifle firmly into his shoulder.

On a service rifle forum I frequent, there is talk from time to time on this subject. Seems a light or soft trigger pull can cause this. As the weapon recoils your finger is still at the point of fire and as the rifle cycles it will fire again.

If it's a CMP SG I guarantee there are no worn parts. Service Grade denotes all parts within spec and battle ready.

It happens often enough to attract many questions on the forum.

The only thing I know for sure to cause that is a worn sear. When the trigger is pulled and the action operates, the sear catches the hammer to keep it from following the bolt forward. If the sear does not catch the hammer, it will fire again without a trigger pull. When it wears enough it will revert to full auto.

Nope

Sounds like it is a issue with a trigger group

M1 Garands are build like a tank a little off lubrication wont make them double fire

Take it to a gunsmith for a 2nd evaluation

Worn sear.... or alight trigger gettin the inadvertent 'double tap'

Grease needs to be used on the op rod and bolt roller.... clp wont stay there