^ That's definitely not true.
After the serotonin is used up (which does enough damage by itself..), dopamine begins to enter the serotonin terminals, causing some major damage. That's why if you keep trying to dose after your roll, you'll only get shitty speedy effects instead of the usual love filled serotonin flood we all love
Also, dosing twice in "quick succession" would be less damaging than dosing once, waiting a few hours and dosing again. In fact, you want to take all of your doses as close together as possible