Two of the same gem on Cospri's Malice?

What would happen if I placed two Frostbolts on my Cospri's?
Will it cast twice as many frostbolts or will it just behave as if I had only one?

multiple copies of the same skill share one cooldown

Thank you.
As a follow up question to what you said- my 6L has COC frostbolt and vortex, so since vortex has a 1.6s cooldown I should not put another vortex on my Cospri's, and instead use a third skill like Ice Nova and such?

I think using a skill with 1.8s cooldown in fast trigger setup is a waste. Its an instant skill so why not using it manually?

frostbolt and ice nova make a good synergy, Id consider them to be a bread and butter for these builds. How many spells to use in CoC 6L setup depends on how fast you are able to hit enemies. one hit (crit) can trigger only one spell from your CoC setup, so two spells are only worth it if you are able to generate two crits (against single target) during the CoC cooldown period.

Wow, I didn't realize you could use vortex manually while cycloning. Will this still be possible in a few hours when Cyclone is channeled tho?

Yes, thats what "instant" skills do. They can be cast without interrupting you. The guard skills, bloodrage and more are pretty nice to use right now.