While insert Record it will display message recordalready exists Visual studio 2005 with SQL Server

Question

While insert same  Record it will display message recordalready exists Visual studio 2005 with SQL Server


textbox1 = Projectcode

textbox2 = Projectname
textbox3 = ProjectDesc
textbox4 = Cost

my code:


protected void Button1_Click(object sender, EventArgs e)
{
con.Open();

str = "insert into Project values('"+TextBox1.Text+"','" + TextBox2.Text + "','" + TextBox3.Text + "'," + TextBox4.Text + ")";

cmd = new SqlCommand(str, con);
cmd.ExecuteNonQuery();

Response.Write("Record inserted Successfully");
con.Close();
}