John, double discharge is the result of something mechanical, something shooter, or maybe a bit of both.
There is no mystery as to why it happens.
Either the gun is to blame, you are to blame, or you are both responsable.

It is REAL BAD NEWS regardless, as it can induce bad flinching and it also leaves you with no second shot.
Your double becomes a single!

If it has never happened before when working up loads, it could mean that you are doing something different in the heat of the moment when a big Bull is in your sights!
Maybe you drank too many "flagons" with Matt the night before!

It could also mean that the wood has swelled or changed slightly in the top end humidity and on some boxlock doubles this can have an affect on the relationship between the trigger assembly and the sear levers.
Also any looseness or movement between stock and action frame can contribute to doubling.

Now is the time that you need a good set of snapcaps so you can try and determine if the gun is still doubling.
Load a live cartridge in the right barrel and a snapcap in the left.
Fire the right barrel and then try to snap the left lock.

Do this lots of times!

If the left lock fails to "snap" it means that it has already been released from firing the right barrel.
This way you can test for doubling without actually risking a true double discharge.

If you don't have snapcaps use a resized primed empty case in the left barrel, and just break the gun after firing the right barrel and see if the primer has been hit and set off on the left.

If after MANY shots with the right barrel, the left barrel always snaps, or the left barrel primer has not been hit, it would suggest that you could have been the culprit.

Either way, to totally put your mind at rest, and restore your confidence, it would be a good idea to have the sears redone and have everything else checked by a good doubles gunsmith, as well as doing the proceedure described above.

Best of luck.

Very nice bull by the way!