DISCLAIMER- NOT FAMILIAR WITH your PARTICULAR SHOTGUN.
Sorry, dang cap lock key-
In general, "doubling" happens when you have worn parts- usually the sear. It is holding back the cocked striker, but under recoil, slips, and the second barrel fires a milisecond after the first.
I know that some of these depend on recoil to move firing to the second barrel. As critical as tolerances could be, and the significant results of a "whoops" when a barrel discharges when you did not pull trigger (like maybe bumping the stock hard) I would be in favor of taking this one to a smith that knows HOW to adjust the parts needed for a SAFE and reliable gun.
Think it was Dirty Harry that said "A man's gotta know his limits."