Is there a way to make a feat or item give proficiency in a skill or double if they already had it?

If you are the DM you can do whatever you want; adding double your proficiency is called expertise. Some classes already get this and yes there are ways both through feats and multiclassing.

I know that i mean for making a homebrew feat or item. The key thing is the item needs to recognise if they dont have it JUST giving them it aand if they do have it doubling it.

When I started DM-ing my campaign, I let all my players choose a free feat since I knew I'd be stealing all their equipment from the start. Dndbeyond doesn't really account for feats at 3rd level, so I made custom "wondrous items" for each of my players. Some just required a text description, but two of my players chose the resilient and skilled feats. This is how you do it:

1. Create a homebrew magic item. I set it to a general item. You'll need to create the item to access more advanced editing.

2. After the initial screen, you will be able to add "modifiers" to your item. From the drop down menu, you can pick "proficiency" or "expertise". The second drop-down menu will then have options for what your item gives expertise in. You'll have to add a new "modifier" for each instance of expertise.

If you need a feat that grants access to spells, you can add this by using the "spell" button to add the desired spell.

I thought if i did that though it would give both not do one or the other.

So I just made a test item to see what would happen.

Proficiency: Adds proficiency bonus to a skill. It does not stack if the character already has proficiency. For example, if the character has proficiency in stealth for a total of +4, this modification does not affect the score. It's still +4

Twice Proficiency: Oddly enough, it does the same as plain ole proficeincy

Expertise: Awards expertise whether or not the character is already proficient in the skill. A level 1 character with +0 in, say, Nature, will then get a +4 with this modification, even though he is not proficient.

It looks like you'll have to tailor the item for each player, adding "expertise" or "proficiency" when needed. Unfortunately, it looks like the system does not have a flat "add proficiency bonus" option right now.

thanks