ASP.NET avoid repeat record in dataset

This is a common and efficient approach in C#:

HashSet processedPolicies = new HashSet();

foreach (DataRow row in ds.Tables[0].Rows)
{
    string policyNo = row["PolicyNo"].ToString();

    if (!processedPolicies.Contains(policyNo))
    {
        // Process this policy
        // Your logic here...

        processedPolicies.Add(policyNo); // Mark as processed
    }
}

SELECT DISTINCT  FROM YourTable