One way to do this is to maintain a list of opened modeless dialogs, and check this list before you open a new one to see if one is already present.

If you open all these modeless dialog's from the same 'main' form, then you can use the OwnedForms property of that main form to maintain this list of opened dialogs. Below are some code snippets that suggest how you must go about this. Note that your dialog forms need to be able to turn off the ownership. This is done below by adding an Owner field to the dialog form.

Sample code that either opens a new dialog or displays an already opened dialog:

private void button1_Click( object sender, EventArgs e ) 
{ 
  foreach ( Form f in this.OwnedForms ) 
  { 
    if ( f is Form2 ) 
    { 
      f.Show(); 
      f.Focus(); 
      return; 
    } 
  } 

//need a new one 
  Form2 f2 = new Form2(); 
  AddOwnedForm( f2 ); 
  f2.Owner = this; 
  f2.Show(); 
} 

Form2 code:

public class Form2 : System.Windows.Forms.Form 
{ 
  private System.Windows.Forms.Label label1; 
  public Form Owner; 
  // ... 

  private void Form2_Closing( object sender, CancelEventArgs e ) 
  { 
    Owner.RemoveOwnedForm( this ); 
  } 
}

Contributed from George Shepherd's Windows Forms FAQ