What Causes Doubling?

[Researcher]

I have a Sterlingworth pin-gun that worked fine if I fired it right/left but would occasionally double when shooting the left barrel first. When the late Bill Harvey disassembled it there was a small splinter stuck on the right sear in old, congealed oil that kept the sear from fully engaging the notch. Gave the right barrel a nice light trigger pull. The stock also had an internal crack so I had him restock it with a piece of Claro I'd been hauling around with me for a couple of decades.