Perhaps the =Unique(A1;A2) function might be helpful to you?
http://woorkup.com/2010/02/19/10-useful-google-spreadsheet-formulas-you-must-know/
Otherwise there was this script from a previous answer on another SE site.
http://webapps.stackexchange.com/a/21699https://webapps.stackexchange.com/a/21699