How to keep duplicates and remove all other items in list?

[heiner55]

#!/usr/bin/python3
list = ['1', '2','2','4', '5','1','6','2', '6']

def clean_up_board(l):
     playable_board = [ ]
     for e in l:
        num_of_occ = l.count(e)
        if num_of_occ % 2 == 0:
            playable_board.append(e)
     return playable_board

print(clean_up_board(list))

Output:
['1', '1', '6', '6']