Delphi : Restore application if already running instead of starting new instance

You might like TLMDOneInstance component, which is free.
All you need to do is just putting component on the main form,

They released LMD-Tools SE 9.04 (LMD 2009)
http://lmdtools.com/download/

Some time ago I wrote similar application. Below is some code of mine.

If you will run this app first time it will just launch. If you will run it second time it will send windows message to previous instance and close itself.
Because I was running my apps with some string parameter, I'm sending this parameter to previous instance of my program too.

Main application dpr file:

program MyApplication;

uses
  Forms,
  Windows,
  Messages,
  SysUtils
  ufrmMain in 'ufrmMain.pas' {frmMain},
  ....;

{$R *.res}

var  
  gMutex: THandle;  
  gString: String;
  gCopyDataStruct : TCopyDataStruct;
  gi: Integer;
  vReceiver: THandle;
begin

  //Prevent before multi instances
  WM_MY_SYNCHRONIZE := RegisterWindowMessage('Some unique windows message name');
  gMutex := CreateMutex(nil, True, 'Unique App Identifier');
  if (gMutex = 0) or (GetLastError = ERROR_ALREADY_EXISTS) then
  begin
    gString := '';
    gi := ParamCount;
    for gi := gi downto 1 do
      gString := gString + ' "' + ParamStr(gi)+'"';
     
    //Searching for main form of previous application instance      
    vReceiver := FindWindow(PChar('TfrmMain'),nil); //TfrmMain is a name of application main form
    if vReceiver<>0 then
    begin
      //Sending message to synchronize data              
      gCopyDataStruct.dwData := WM_MY_SYNCHRONIZE;
      gCopyDataStruct.cbData := 1 + (Length(gString) * SizeOf(Char)) ;
      gCopyDataStruct.lpData := PChar(gString) ;  

      SendMessage(vReceiver, WM_COPYDATA, Application.Handle, Integer(@gCopyDataStruct)) ;        
    end;
  end
  else
    try
      Application.Initialize;
      Application.MainFormOnTaskbar := True;
      Application.CreateForm(TfrmMain, frmMain);
      Application.CreateForm(  ...  );
      ....
      Application.Run;
    finally
      if gMutex <> 0 then CloseHandle(gMutex);
    end;
end.

My main form unit
unit ufrmMain;

interface

uses ... ;

TfrmMain= class(TForm)
...
private
  ...
  procedure SynchronizeMessage(var Msg : TWMCopyData) ; message WM_COPYDATA;
end;

var
  frmMain: TfrmMain;
  WM_MY_SYNCHRONIZE: LongWord;

procedure TfrmMain.SynchronizeMessage(var Msg: TWMCopyData);
var
  vParams : string;
begin
  if Msg.CopyDataStruct.dwData=SYNCHRONIZE then
  begin
    vParams := PChar(Msg.CopyDataStruct.lpData) ;

    And here you can restore this instance of application
    for example:

    if isIconin(Application.Handle) then
    begin
      Application.MainForm.WindowState := wsNormal;
      ShowWindow(Application.Mainform.Handle, sw_restore);

      hCurWnd := GetForegroundWindow;
      AttachThreadInput(
        GetWindowThreadProcessId(hCurWnd, nil),
        GetCurrentThreadId, True);
      SetForegroundWindow( Application.MainForm.Handle); //there should be a handle to window, which should be show on top (currently active windows of yours app).
      AttachThreadInput(
        GetWindowThreadProcessId(hCurWnd, nil),
        GetCurrentThreadId, False);
    end;
 
 end;
end;

Hmmmm
I suppose that if you would delete line:
if isIconin(Application.Handle) then
from my code then it should work fine, my mistake.

PS: From time to time, if you are doing such things, windows which are showing, are not on the top of all others windows on the desktop. This happens only sometimes and in most cases can't be propagate so I advice you using below code in OnFormShow event which prevents such behaviour.

  hCurWnd := GetForegroundWindow;
  AttachThreadInput(
                    GetWindowThreadProcessId(hCurWnd, nil),
                    GetCurrentThreadId, True);
  SetForegroundWindow(Handle); //Current window handle
  AttachThreadInput(
                    GetWindowThreadProcessId(hCurWnd, nil),
                    GetCurrentThreadId, False);

Select allOpen in new window

Yeah I supposed that isIconin = IsIconic when I tested your code :)