Python: How to find duplicates in array

I'm new to Pyton, and is trying to define an array by numpy and is hoping to list out duplicates in the array

a = np.array([[1,2,3],
             [4,1,6],
             [2,3,9]])

 def arrayDup(a):
    size = len(a)
    for i in range(size): #row
      for j in range(i+1,size): #col
        if a[i] == a[j]:
            print(a[i], end='')
arrayDup(a)

The duplicates are 1,2 and 3, the code that I have above does not seem to print out the duplicated values, and I cannot find the source of error.