The condition is called sear reset failure and is VERY dangerous. Please do not fire that rifle or allow it to be sold until a gunsmith can fix it.
I will add to what Dfariswheel said by noting that in the vast majority of those cases, the condition is not caused by either wear or parts replacement; it is caused by someone determined to stone, file and grind in an attempt to "improve" the trigger pull. When he finds that he has made the gun dangerous he doesn't spend money to fix it, he trades it off so some poor sucker will get sued when the gun blows someone's head off.
Jim
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