Leica SF20 flash fires when advancing film?

I just bought a SF20 Leica flash from KEH. Put fresh batteries inside and tried it out....it fires fine when I click the shutter but it fires a second time when I advance the film. Thought it might be a problem with my M6TTL but I popped an old Pentax flash on and that worked fine + didn't fire a second time.

This is my first attempt at using a flash with my Leica, so is the SF20 most likely broken? Or am I not doing something right with it?

The M6 TTL has four contacts; the old Pentax flash might not be using all of them.

To ensure it's not a vibration or looseness issue, place the M6 on a table, hold it down, and slowly, smoothly wind it. If it flashes, I suspect the the trigger signal is indeed coming from the M6.

The only other thing I can think of is that maybe the flash's shoe isn't fully inserted.

Thanks for the reply. I took another look at the flash and noticed that the advance lever only triggers the flash while the red "OK" exposure light is illuminated on the flash. This is only for 3-4 seconds after the flash is first triggered by the shutter. When the red "ok" light disappears after 3-4 seconds, I can advance the film without triggering the flash again.

The Pentax flash I was using only had 3 contacts, so I guess it could be an issue with the contacts on the Leica flash. I'm sending the flash back to KEH just to be safe though.

Clean the contacts with a new pencil eraser, then blow off. Try the table suggestion above too. Contact KEH if it continues.