10 gauge lifter double firing--both hammers fall

[B. Dudley]

As dean said, the trigger pull on both triggers is way too light. the recoil from the first shot is setting off the second. Both sears need reworked to firm up the trigger pulls and correct the issue of doubling.

[edgarspencer]

Hammer springs on lifters, and top lever hammer guns made by Parker, were always stout. These springs play, in part, to trigger pull. Lightening the springs for ease of cocking both hammers with the trigger hand thumb is something hunters learn to do early on when hunting with a hammer gun.
While there are many accomplished gunsmiths, not all are well versed in spring work, and lightening and polishing a hammer spring is not a job for just anyone.
Light framed guns shooting moderate to heavy loads need to have trigger pulls set accordingly; generally 4 lbs, or more.
With the exception of my skeet gun, I have only shot double trigger guns, and have learned to like my left barrel trigger adjusted to a half pound heavier than my right barrel. (3.5 lbs right, 4 lbs left)