Rerolling skin shards

Hello, I was wondering if any of you would have any deep knowledge regarding rerolling skin shards. You see, I'm usually lucky regarding the value of most skins that I get from normal chests to orbs and such from different events. Altho, especially recently i noticed something in the rerolling of my skins. You see, I personally prefer to disenchant most of the skins I get, because, like i previously stated I can consider myself 'lucky' when it comes to Hextech Crafting. The thing is that on the last rerolls, when I reroll, i get skins of owned champions on which I already have a skin in my Possession. Let's say I made a skin permanent, the SKT one, last month, 1 week ago i rerolled and got Academy Ekko, which i don't really need. On i already had 3 skins, when i rerolled 3 skin shards and got Kitty Kat one. Last week i buyed from the store, i made the Galactic skin permanent and on my last reroll, from today, i got Pool Party ( i understand that it is nicer but it's still my 2nd skin for a champion I just bought and i don't need it). This has also happend to a gragas skin 2 months ago. So, from 5 rerolls, 4 were for Owned champions on which I already got at least 1 skin. Is this tough luck or this is how the rerolling system works lately?