I have month old Blackwidow v4 75%, and a handful off keys have started double typing. This is not an uncommon issue, and I see regular posts about it here.
Currently it is my F/L/E keys, mostly. Sometimes other keys have it happen.
I have tested with Synapse disabled, and I have cleaned out beneath the cap - I have NOT removed the switch itself and cleaned under that.
Usually this seems to be a Firmware fix, but there is now firmware available for this keyboard (yet).