How do you find most duplicates in a 2d list?

I have a 2d list that i would like to return the most duplicates by using a list comprehension. For example, i have a list below

a = [[10, 15, 17,],[20,21,27],[10,15,17],[21,27,28],[21,27,28],[5,10,15],[15,17,20]]

I would like my result to be

b = [[10,15,17],[21,27,28]

One line splitted here:

[ a[k] 
  for k in range(len(a)) 
  if  a.count( a[k] ) > 1
  and k == a.index( a[k] ) ]