How to make multiple checkbox add to database

Question

I need to add more one selected item from checkbox to database but when I select more one I could not for one Item it works fine 

 this the code

 

please help

thanks  

cmd.CommandText = "If Not Exists(select * from dbo.ModuleRelation where UId=@UId and MId=@MId) Begin insert into dbo.ModuleRelation (UId, MId, MName) values(@UId, @MId, @MName) End";
               cmd.Connection = conn;
               foreach (ListItem item in chkModules.Items)
                   if (item.Selected == true)
                   {
                       conn.Open();
                       cmd.Parameters.Clear();
                       cmd.Parameters.AddWithValue("@UId", DropDownList1.SelectedItem.ToString());
                       cmd.Parameters.AddWithValue("@MId", item.Value.ToString());
                       cmd.Parameters.AddWithValue("@MName", item.Text.ToString());
                       chkModules.Visible = false;
                       cmd.ExecuteNonQuery();