I don't know if it is relevant, but I had a similar situation with my 625.
Kept track of moonclips, headstamp, etc., no common thread.
HOWEVER, it seemed to happen with light loads and long shooting sessions.
Best I could figure, the light loads didn't expand enough, and I got fouling that (slightly, almost imperceptibly) impeded chambering. So the firing pin strike just finished the chambering. The 'light strikes' fired on second attempt.
I now just brush the chambers (dry) when I'm shooting high round counts of light loads, and I haven't had a recurrence.
Hope it helps, if it does thanks goes to the guy shooting a 686+ that told me about his similar issues.
John