When I upload data that time duplicate data occured notice .who your data duplicate

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C#

private void btnUploadData_Click(object sender, EventArgs e)
        {

            try
            {
                bool check = validationUpload();
                if (check == false)
                {
                   // MessageBox.Show("invalid entry");
                    return;
                }
                else
                {

                    string connectionstring = "Data Source=10.10.5.251;Initial Catalog=Demo;User ID=dev;Password=SamDev@2020";
                    DapperPlusManager.Entity<UploadSBAPayment>().Table("UploadSBAPayment");
                    List<UploadSBAPayment> SBAPayment = uploadSBAPaymentBindingSource.DataSource as List<UploadSBAPayment>;
                    if (SBAPayment != null)
                    {
                        using (SqlConnection con = new SqlConnection(connectionstring))
                        {
                            con.Open();
                            con.BulkInsert(SBAPayment);

                            con.Close();
                        }

                    }
                }
                    MessageBox.Show("Data Upload Successfully");
                
                
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "Message", MessageBoxButtons.OK, MessageBoxIcon.Error);

            }          

        }

What I have tried:

i have try C# upload excel file load to gidview then after gridview sql table that time if duplicate data notice to user ...................................................