With a Vial I obviously prefer to use a fresh pin to inject with, but have used the same pin on a vial and then myself with no negative results. Mind you I again point out I prefer to use a fresh pin.
With an amp, i would NEVER use the same pin to inject with.
You are pressing the pin to the glass (in an amp) and holding the amp on an angle. Unless you have some other way or a 3rd hand, I hold the amp in left hand use my left hand fingers to hold the barrel, and my ring hand to draw. I know the pin scrapes on the glass of the amp and would obviously be very blunt and painful if i wanted to use it to inject.
On the other hand a vial unless down to the end, my pin doesnt touch the glass. It will not be as sharp after pricking the skin of the vial, but it isnt as blunt as if it touched or rubbed against the glass.

Hope that helps. And if it doesnt, let us know how the injection went. PS - youre up to date with your tetanus shots, right?