JM at work,
Steve Smith has the right of it in this case.
Shooting an M1 or M14 from a bench with the butt loose in your shoulder can lead to "bump firing" or doubling. Mechanically the rifle is functioning perfectly normal, it is only reacting to recoil and firing again as it bounces off of your shoulder and back into your trigger finger.
This will stop if you just have the butt firmly into your shoulder when you fire from a bench. It also helps to have a firm follow through on your trigger squeeze.
If you do get it firm into your shoulder and it still happens frequently, then I'd have a good M1 smith check your trigger group for worn sear and hammer...
Re Slam Fire: This is definitely a concern with the M1-M14. Making sure you have fully seated your primers is mandatory for safe handloads with these rifles. Also part of this equation is full length resizing of the brass and keeping the chamber clean.
I seriously doubt that what you experienced was a true slam fire. A true out of battery slam fire would REALLY get your attention as this usually happens before the bolt is fully locked up. If an out of battery slam fire does occur pieces and parts of the rifle can and do "go ballistic".... The firearms equivalent of an Indy car after a header into the wall. Not pretty and seldom happens to the same rifle twice.
Garands forever,
Swampy