comparing the values in datagrid

I've to compare the values of the columns in grid view to make the values in the first column unique. The values are stored in ViewState["CurrentTable"] in code behind page. Suppose

COL1

JAN

FEB

MAR

APR

Since the rows are editable the user might re-enter duplicate values but I've to prevent this too. How can I check the values in the column to prevent duplicate entry? Please suggest.