I'm making a subclass, and I want one of it's 3rd level abilities to be like the Gloomstalker Rangers "Umbral Sight", but with a Skill Proficiency instead of Darkvision. Ie, "When choosing this subclass, you gain Skill Proficiency, but if you have it from another source, you gain double proficiency." but l have no idea if that's possible with the homebrew maker. If it is, please tell me, otherwise i'll have to note that the double proficiency would have to be added on the Sheet by the player.

The best you could do is make two Options, one for the proficiency, and the other for the Expertise. (Use Expertise, not “Twice Proficiency.”) 

Oh, that's a good idea, thanks! Though any spesific reason it should be Expertise?

If you use Twice Proficiency it will override their base proficiency and allow them to pick a new proficiency to replace it as per the rules about gaining the same proficiency from multiple sources. Expertise will simply double their preexisting proficiency.

Ah, good to know, thanks!