How to check duplicate data on Google Spreadsheet. You can do it this step.

1. Select Column
2. Choose Format - Conditional Formating
   
   
3. in Conditional Formating Choose formula cell if custom 
4. put the code
   

   =countif(A:A,A1)>1

5. A is column A, you can change the column
   
6. and the duplicate data will be highlight with color you choose