มีวิธีเช็ค Username ห้ามซ้ำไหมครับ ?? C# + SQL Server 2008

			int intNumRows;
			strSQL = "SELECT COUNT(*) FROM customer WHERE CustomerID = '"+ this.txtCustomerID.Text +"' ";
			objCmd = new SqlCommand(strSQL, objConn);
			intNumRows = Convert.ToInt32(objCmd.ExecuteScalar());

			if(intNumRows > 0)
			{
				this.pnlAdd.Visible = false;
				this.lblStatus.Visible = true;
				this.lblStatus.Text = "CustomerID already exists.";
			}
			else
			{
				strSQL = "INSERT INTO customer (CustomerID,Name,Email,CountryCode,Budget,Used) " +
				" VALUES " +
				" ('" + this.txtCustomerID.Text + "','" + this.txtName.Text + "','" + this.txtEmail.Text + "', " +
				" '" + this.txtCountryCode.Text + "','" + this.txtBudget.Text + "','" + this.txtUsed.Text + "')";

				objCmd = new SqlCommand();
				objCmd.Connection = objConn;
				objCmd.CommandText = strSQL;
				objCmd.CommandType = CommandType.Text;
								
				this.pnlAdd.Visible = false;

				try
				{
					objCmd.ExecuteNonQuery();
					this.lblStatus.Text = "Record Inserted";
					this.lblStatus.Visible = true;
				}
				catch (Exception ex)
				{
					this.lblStatus.Visible = true;
					this.lblStatus.Text = "Record can not insert Error ("+ ex.Message +")";
				}
		}

int intNumRows;
                string sql = "SELECT * FROM TB_Admin WHERE Username = '" + textBox1.Text + "'";
                cm = new SqlCommand(sql, cn);
                intNumRows = Convert.ToInt32(cm.ExecuteScalar());
                if (intNumRows > 0)
                {
                    MessageBox.Show("Username " + textBox1.Text + " มีอยู่ในระบบแล้ว", "พบชื่อผู้ใช้งานซ้ำ", MessageBoxButtons.OK, MessageBoxIcon.Error);
                }