Suspect it is all part of the same problem.
We have no other reports of dual vmails. This is a pure Asterisk function; SAIL doesn't have much to do with it other than invoking vmail after the ring times out. Also, in voicemail headers, there is a minmessage setting which, from memory, is set to 8 seconds. This would force Asterisk to discard anything shorter than 8 seconds so you may want to check that setting also.
Best
S