Just to add on to Circuits post, since the rifle is used and running a two stage FCG, The area to concentrate on will be the disconnector and corresponding hammer rear hook.
The simplest way to attack the problem after function check is to just confirm that the all the sear surfaces/ledges are still sharp (not screwed up by a novice trying to re-work the trigger), and that the disconnector spring was not clipped (someone trying to wrongly lighten the secondary pull after disconnector engagement).
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To walk you threw the FCG, since the two-stage triggers are not listed in the manuals,
At trigger rest/ hammer set, the front sear of the hammer is retained by the front sear of the trigger. As you start to pull back the trigger, the back hook of the trigger (disconnector) presses against the rear hook of the hammer (above the sear surface). This engagement is the secondary pressure of the trigger that you feel just before the hammer releases. On a stock trigger, you will have some creep after the extra pull weight, which is a safety factor in order for the FCG to work in a wide variety of rifles with out needing to be smithed/ tuned from the start. This added creep factor is that in order for the hammer to make it’s way threw the front and back hook to be released, the trigger must be fully released in order for the hammer to be released from disconnector hook, caught by the primary sear (front), then the trigger needing to be pulled to release the hammer off the front hook (with tension from the back hook adding trigger pressure).
When doing a trigger job on a two stage FCG, the disconnector is worked to so that once the hammer disconnector sear touches the rear hook (above the hammer sear) the next movement of the trigger, with the added tension of the disconnector, should be the trigger/hammer releasing (like breaking a thin glass rod). Polishing/stoning the sears to set the FCG to a specific rifle that it is being used in will accomplish this ideal functioning of the FCG. Ideally, the first stage of the trigger should be around a 2 lb with creep of about ¼”, then the 4.5 lb resistance of the secondary pull weight (no creep just added resistance of the disconnector) with the clean break of the hammer.
Now if someone tried to do an untalented trigger job on the FCG and screwed it up, they may have rounded the sear edges, then set the disconnector with not enough retention. This lack of retention would be that the gap between the trigger hooks would be set too wide, and the hammer can be danced out of the trigger sears with just the slightest of trigger dancing (trigger finger movement at recoil bouncing on the hammer). Or, they may have stoned too much surface off the disconnector sears, and the FCG lacks any rear two stage effect (hammer can make it way out of the hooks with not trigger movement due to the hammer disconnector sear slipping past the disconnector sear if the trigger is held in just the right position.
To sum up this post, I could have just told you from the start that you need to take the rifle to a smith to check it out (rework the FCG if possible), but like the rest of us, you want to fix the problem yourself. If the fire control group (FCG) were just the normal unit of a standard rifle, this would not be a problem and most of the guys here could instruct you on adjusting/tuning a standard disconnector. But, since the trigger group is a two-stage unit, and takes a talented smith to set one up correctly (match tuned, and not just sloppily functioning), chance are you may total destroy the FCG in the process of trying to correct it (hacked trigger job). My guess is still that the disconnector is set incorrectly, and hopefully you can find a local smith that can correct the problem with out needing to replace the entire trigger/hammer group.