comparing the values in datagrid

Question

I am using C# / ASP.NET 

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.