I admit, I'm not familiar with the term "double tap", so I had a look at wikipedia, which stated that it is something like a rapid double shot.
If it is meant to trigger once but shoot twice within a certain timespan, than you can do it this way:
var 'double' - 0 for single shot, 1 for double
var 'time' - timespan between first and second shot in milliseconds
var 'timestamp' - (make this a private var to the bullet object, initially 0) the moment when the first shot is activated
+ some condition that activates double shot
-> 'double' = 1
+ key for shooting pressed
-> spawn bullet
++ if 'double' == 1
--> bullet('timestamp') = timer
+ bullet('timestamp') > 0
++ bullet('timestamp') <= timer - 'time'
--> spawn bullet
# not from this bullet but from the player!
If this isn't what you're looking for, please ignore or explain in detail