Since you can get skill-ups of different magnitudes (for example a 0.1 and a 0.2), I think it's pretty much doubles the probability of getting a skill-up. So yeah, it doubles the number you get.
Pelican ring stacks. It increases the chance of getting a skill up. There's also a chance of double and triple skill ups from having 2 ring enchants on. Check the fishing thread on here.
Pelican ring actually lets you get double skill ups. Like if you get a .2 you will get another .2 from it. It doesn't increase the amount of skill ups you get.