var MyForm : TChildForm;

procedure TChildForm.OnDestroy;
begin
  MyForm := nil;
end;

Here are a couple of routines from an application we wrote, which as it
happens does not allow more than one instance of a form-class to exist
(this by choice), but which does illustrate the type of work that is
involved.  This app used resource-strings for all messages, and has some
utility-routine calls that are specific to our work, but you can see the
essential idea.

<snip!>

{
  Baby-making.  :-)
  The main form is an MDI-child application.  The "MakeAnotherChild"
procedure
    reproduces indiscriminately (ahem...), while "ActivateOrMakeChild"
searches
    to see if a child-window of this type exists.  If it does, the
existing child is
    brought front-and-center.
  By design, multiple instances of child-forms are never allowed to
exist.
    (We'd have to have multiple instances of the data-model, for one
thing,
    and we'd confuse the hell out of users, for another...)

function TMainForm.ActivateOrMakeChild(ofClass: TFormClass) : TForm;
var
  n:  integer;
  f:  TForm;
begin
  { Search for the most recently created form of the specified class }
  f := nil;
  for n := MDIChildCount - 1 downto 0 do begin
    if MDIChildren[n] is ofClass then begin
      f := MDIChildren[n];     { found it - note and break out of loop }
      break;
    end;
  end;
  if Assigned(f) then begin            { found it? }
    F.BringToFront;                    { make it come to the front }
    if F.WindowState = wsMinimized
      then F.WindowState := wsNormal;  { and be sure it's large enough
we can see it }
  end else begin
    Application.CreateForm(ofClass, f);
    if f.FormStyle <> fsMDIChild then begin   { common mistake }
      SayStop(FmtLoadStr(M_FormNotMDIChild, [f.ClassName]));
    end;
  end;
  result := f;
end; {ActivateOrMakeChild}
</snip!>