Prevent repeat printing of common list variables Haskell

Question

Currently, I am making a function that can take the values in two lists, compare them, and print any similar values. If a value is duplicated in both lists, it is not printed a second time.

EXAMPLE

INPUT: commons [1,2,2,3,4] [2,3,4,5,2,6,8]
EXPECTED OUTPUT: [2,3,4]

What I currently have causes a repeated values in both lists to repeat in the output. So, in the above example, the 2 would print twice.

Here is the current code I am working on:

commons :: Eq a => [a] -> [a] -> [a]
commons [] [] = []
commons x y = commons_Helper x y

commons_Helper :: Eq a => [a] -> [a] -> [a]
commons_Helper [] [] = []
commons_Helper x [] = []
commons_Helper [] y = []
commons_Helper (x:xs) y = 
    if elem x y then x : commons_Helper xs y
    else commons_Helper xs y 

Any and all help would be greatly appreciated.

EDIT: This must remain as commons :: Eq a => [a] -> [a] -> [a], and I cannot import and libraries