The first strike is a warning and the second is a bite - usually..
..in the case of humans encountering an agitated,startled or agressive venomous snake. They do not typically waste their venom on a creature that is not a suitable meal. Just as rattle snake will use its rattling tail to ward off danger and if that doesn't work, it will strike.
Don't even put this to the test because you have no idea what the snakes view is, it may have already issued you a warning but you did not see it. It can have other ways of warning such as rearing up. People can also be bitten but not necessarily injected with venom.
The snake in the photo above is not venomous but she can still bite and has a mouthful of many teeth used to grasp her prey . She will only bite to defend herself or a clutch of eggs, she will otherwise not attack a creature that is not a meal.