Release the left windows key within 0,3 seconds after pressing it, to do something else (e.g. to send a):
~LWin::
KeyWait, LWin
return
~LWin Up::
Send {LWin Up}
If (A_PriorHotKey = "~LWin" AND A_TimeSincePriorHotkey < 300)
Send, a
; else ; another action after long press (not recommendet)
; Send, b
return
EDIT:
Try also this:
LWin up::
If (A_PriorKey = "LWin")
Send a
return
; In this case its necessary to define a custom combination by using "&" or "<#"
; to avoid that LWin loses its original function as a modifier key:
<#d:: Send #d ; <# means LWin