You misunderstand, or would for some reason be carrying a DA or DAO unchambered for that to happen. Chambered DA or DAO will fire on the first pull, and DA will be single-action trigger after that first shot.
Glock is not SA - the act of pressing the trigger cocks the striker, which is at rest and loose when the trigger is forward (shake one sometime and you can hear the firing pin move a little). They do lack full DA "second strike" capability, since they require slide movement to reset the disconnector after a trigger press, though.(Actually, to quote wikipedia, "Glock, Springfield XD and XDm, and Kahr semi-automatic pistols are not DA (or DAO) pistols because the striker is "cocked" to an intermediate position by the operation of the slide and they cannot be re-activated by pulling the trigger a second time." I would add a lot of other models to that list too, such as the lauded LCP, if I recall correctly.)