I think the RNG of hextech is broken. I keep getting doubles of at least 3 skins now.

Not really broken, it was just never honest to begin with. The odds of getting a particular shard three times in a row are roughly 1/1,000,000,000 (there are around 1,000 skins in the game, so 1/1,000 x 1/1,000 x 1/1,000).

I'm totally convinced there's hidden aspect to the formula that increases the odds of duplicates by an insane amount.

Duplicates are surprisingly common. The problem is called the birthday paradox in probability theory if you're interested. tl;dr: In a room of 23 people there is a 50% chance at least two will share a birthday. In a room of 70 people it's a 99.9% chance.

If we assume there are 900 skins you can get through Hextech crafting, you are more likely to have a duplicate than not, once you get more than 35 skins/skin shards. At 23 skins it's a 25% chance, at 14 it's a 10% chance. Skins you have bought outside of Hextech Crafting further increase that probability.

If anything it would be an indicator for broken rng if you weren't getting duplicates.

The problem with randomness is that you technically can't tell whether it's broken through mere observation.