Haven't heard of a penis hurting from back to back masturbation... however, the prostate gland, deep in your groin, could be considered to be part of the root of the penis. It can ACHE from over-use, especially when back-to-back ejaculations are triggered.
Now, if you're gripping or abusing your penis during your masturbation by being too rough, then that could cause some pain, especially if you have some time for feeling to return between sessions.
But if you're talking about the glanz or head of the penis, it becomes supersensitive any time climax in neared. And that is normal. It will fade with time.
No, but like any muscle, it gets tired from over-use. It needs to rest.
Let it rest up for a day or two between your sessions. At least, I find that's enough for me.
You may be a bit more resiliant, and able to handle it again sooner.
But no, you can't "damage it" from orgasm. It just spasms and pulses about ten or so times until it's "spent".
Relax.