This is one of those classic examples of a “Roof in a Can”.
Roof in a Can is a term we came up with when describing a repair job we’ve seen; it’s when the person working on the roof can’t figure out the right way to do something so he just squirts caulk over the problem. I have a collection of these pics somewhere, I’ll put them up someday.
Before:

These plumbing vents are really close together; too close for the installer to use two pipe collars. The pipe collar is installed on the pipe on the left, the pipe on the right got the Roof in a Can treatment.
I would love to know how the person decided which pipe got the collar and which one got the caulk. Flip a coin? Did he ask a coworker? Did he think the larger pipe was mocking him? Maybe he just wanted the smaller pipe to have the collar, pulling for the underdog on the roof.

This took me about thirty minutes; it’s made out of 16oz copper and will last for the next three roofs. No caulk to dry out and crack, just copper and lead solder. Solid.
There they stand ready to face the weather again. Both pipes treated with practical dignity; flashed with copper and installed with pride.
Caulk is not a roof and caulk doesn’t last, don’t let someone “repair” your roof with a caulking gun.
If you have leaking pipes on your roof give us a call.
Tom