For an ordered list, remove permutations and non-unique elements & preserve order

gistfile1.py

from itertools import permutations
def ret_unique(a):
	'''returns a copy of the list with:
		order preserved,
		permutations removed,
		repeat elements removed'''
	for i,el in enumerate(a):
		b = permutations(el,3)
		for el1 in b:
			if list(el1) in a[i+1:]:
				del a[a.index(list(el1))]
			if list(el1) in a[:i-1]:
				del a[i]
	return a
a=[['a','b','c'],['d','e','f'],['b','c','a'],['f','d','e'],['a','d','f'],['a','b','c'],['d','e','f'],['b','c','a'],['f','d','e'],['a','d','f']]

print ret_unique(a)

[['a', 'd', 'f'], ['a', 'b', 'c'], ['d', 'e', 'f']]