testing a sequence for 'identicalness'

[Rajarshi Guha]

Hi,
  does anybody knwo how I can check a sequence for 'identical ness', ie
given a sequence:

[1,1,1,1,1,1,1,1,1,1]

it would be classified as 100% identical

and

[1,1,1,1,1,1,1,1,1,2]

would be classified as 90% identical.

TIA,