What Pat said. If it doubles or fanfires after it is cleaned and inspected, let some one else shoot it. If it does not double for them, you are fan firing the gun, and need to hold it more securely. If it doubles for another shooter, you need to have it checked out.
Most O/U's can be made to fan fire by not holding them securely. (ouch)
The hammer knoch and the surface of the sear which engages the hammer knoch need to be cleaned of all lubrication. Use a cue tip or pipe cleaner dipped in rubbing alcohol to clean the surfaces. This should solve the problem if nothing is broken or damaged. HMB
I had friend who shot a Ljutic o/u. It had always been reliable and he shot it great. One day during registered doubles it doubled. Back then you could test fire. He test fired twice more and the gun doubled. He finished with a Winchester SX-1.
I saw him a week later and asked about his problem. He told me there was nothing wrong with the gun but he had been using WD-40 internally when he cleaned the gun. This he had done several times. The gunsmith told him to never us WD-40 inside the workings as WD-40 leaves a film that gets gummy over time especially if its used over and over.
What was happening in his case was the build up was preventing the inertia block from going full stroke.
Many shooters don't realize that when you pull the trigger on a over and under you actually pull it twice. Once when you fire the gun and once when the gun comes back from recoil and runs into your finger. That's why there is an inertia block to interrupt this cycle. If the block can't go full cycle it can't do it's job.
When shooting a single barrel the above phenomenon doesn’t happen because you are only shooting one shot.