facing issues with unique and dropDups

My db is having a collection having documents like:
{
  _id : ObjectId(.....)
  a : 1,
  b : 2,
  c : [{a1 : 2, a2 : 4}, {a1 : 3, a2 : 1}, {a1 : 6, a2 : 7}]
}

I want to have a unique value for 'c.a1'. My collection design is such
that each time I have to push {a1, a2} pairs in 'c'
To make sure that I have unique value for 'c.a1' I have created
index :
{
		"v" : 1,
		"key" : {
			"c.a1" : 1
		},
		"unique" : true,
		"ns" : "some_db.some_collection",
		"name" : "c.a1_1",
		"dropDups" : true,
		"background" : true
}

but still I am able to insert, same value for 'c.a1' i.e. {a1 : 1,
a2 : 10}, {a1 : 1, a2 : 11} , both are getting inserted ??
Logically the second one should be dropped if first one already
inserted because of unique index on 'c.a1' ?

I am using mongo 2.0.0 version