[lmikle]

Например, так:

Код:

function GetFileName(AFileName : String) : String;
var
  I : Integer;
begin
    I := 0;
    Result := AFileName;
    While FileExists(Result) Do
      Begin
        Inc(I);
        Result := ChangeFileExt(AFileName,'('+IntToStr(I)+')'+ExtractFileExt(AFileName));
      End;
end;