CodeSignal First Interview Problem

1.py

import numpy as np
def firstDuplicate(a):
    
    num = -1
    
    
    seen = np.zeros(len(a))
    for x in a:
        if seen[x - 1] == 0:
            seen[x - 1] +=1
        else:
            num = x
            break
        
    return num