integers.py

def get_unique(m:list): 
     c = {} 
     for i in m: 
         if c.get(i) is not None: 
             c.pop(i) 
         c[i] = i 
     return next(iter(c.values()))
    
 assert get_unique([6,1,3,3,3,6,6]) == 1
 assert get_unique([13,19,13,13]) == 19
 assert get_unique([1,2,3,4,5,1,2,3,4,1,2,3,4]) == 5

this works it's O(1) space and O(M) time.
Try using the bitwise operators to achieve the O(1).
I did my logic in Golang. here's the link