How can I check for duplicate records from my datatable to my database records in SQL server?

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C#

SQL

database

server

DataTable

I'm using datatable to insert records to my database in sql server and the reason why I'm using datatable is because the records to be inserted is on the datagridview. my question is, how can I check or validate records just in case I accidentally put duplicate records inside my datatable before saving it to the database. pls. help.thank you

What I have tried:

i have tried this but it still accepts duplicate records.

SqlConnection connectionstring = connectionString.Getconnection();
SqlCommand command = new SqlCommand();
using (command = new SqlCommand("sp_Product_Name", connectionstring)//how to call stored proc with SELECT command
{
    CommandType = CommandType.StoredProcedure
})

   command.Parameters.AddWithValue("@SI_no", txtSalesInvNo.Text.Trim());
    command.Parameters.AddWithValue("@prod_name", data);
int foundrec = Convert.ToInt32(command.ExecuteScalar());

if (foundrec == 0)
{
    Save_SalesInvoice();
    foreach (DataGridViewRow row in dgvCustProducts.Rows)
    {
        dgvCustProducts.Rows[row.Index].SetValues(false);
        row.Cells[1].Value = 0;

    }
}

else
{

    MessageBox.Show("One of the Products already exist. Please Check the Records", "Duplicate Records", MessageBoxButtons.OK, MessageBoxIcon.Error);
    //txtSalesInvNo.Text = "";
   //txtSalesInvNo.Focus();

}