Jack is the face of Boe and has lived for millions (or billions) of years; The Doctor has called him a fixed point in time and space. In the short of 'time crash' the paradox of meeting himself and the Tardis' colliding would destroy a portion of the universe the size of Belgium. The Doctor is a very important part of the universe, but Jack being a "fact" is also very important. What would happen if he time traveled and met himself long enough to create a paradox? Canon if possible
The best answer you will get is that whatever the writers want to happen will happen. There are a veritable crap-ton of paradoxes in Doctor Who (and I've only watched the reboot), and none of them are handled the same way.
In Father's Day:
Rose saves her father, creating a paradox, and then somehow creates a bigger one by touching the baby version of herself. In this case, Reapers came to "fix" the problem.
In A Good Man Goes To War:
We learn that Amy and Rory named their child after their best friend from childhood who was their child. Nothing happens.
In The Hungry Earth:
Amy and Rory wave to each other from a distance, even though that should not be possible since Rory is later erased from time.
And then there's the whole Clara thing. Basically, it is impossible to know, because Doctor Who is invariably inconsistent.