How do I verify existing couples inside an array?

Hi, i'm buiding a Card game. I have an array 4x2 containing the UID of the card on first row and a value on second one.

I should verify if there are couples or more inside the array and then retrieve the relative UID. For example

1 2 3 4

11 11 5 6

this should pick 1 and 2 UID

1 2 3 4

11 11 8 8

Shoud pick all cards and

1 2 3 4

11 11 11 6

first 3

What woul be the best way to do it without a lot of useless and messy code that i would use for sure now?

Thanks for help!