Hello!
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Duplicated skills are literally copies that you can place other perks of the skill that was copied. They share the cooldown.

For example:
You can double the winter grasp, and in the first, put ice damage and in the second, shadow damage.

havent seen any1 utilize this, so cant be very good

you can use different skills which don't have a cooldown - i didn't try it out very much on my own because my main wouldn't need it but like 2 solarfalls would be interesting - one stationary and one channeled or stuff like that.

You can also use it for minions to have 3x livor mortis - it works i just tried out

Thank you all for your help!