When I do something like that, I always call the pharmacist (Wal-greens, Eckerd, Rite-Aid) whichever one, and I ask to speak directly to the Pharmacist and I discuss with them what I did and what I should expect. I think you should try that. I have never took 2 doses of phentermine, but I have other things, like NyQuil along with Sudaphed; and other things.
Let me know what happens. I always get scared too!!