Remove duplicated lines from file working, but not displaying correct index

The following code works fine removing correct duplicated lines, but when i show I before remove the line of StringList is displayed correctly the index (I) only of the first ocurrence of duplicated, after, the index is showed wrong not corresponding to correct (always minor than real index). How fix it?

This is the code:

program Project1;

{$APPTYPE CONSOLE}

{$R *.res}

uses
  System.Classes,
  System.SysUtils;

var
  slSrc, slDup: TStringList;
  I, J: Integer;

begin
  try
    slDup := TStringList.Create;
    try
      slSrc := TStringList.Create;
      try
        slSrc.LoadFromFile('C:\src.txt');

        for I := 0 to Pred(slSrc.Count) do
        begin
          for J := Pred(slSrc.Count) downto I + 1 do
          begin
            if SameStr(slSrc[I], slSrc[J]) then
            begin
              // Ex; If 1º dup index is = 3, is showed 3. OK
              // If 2º dup index is = 15, is showed always a minor like 13 for example
              // and continue displaying wrong index for next dups

              Writeln(IntToStr(I) + ' : ' + slSrc[I]);
              slDup.Add(slSrc[I]);
              slSrc.Delete(I);
            end;
          end;
        end;
        slSrc.SaveToFile('C:\src.txt');
        slDup.SaveToFile('C:\dup.txt');
      finally
        slSrc.Free;
      end;
    finally
      slDup.Free;
    end;
  except
    on E: Exception do
      Writeln(E.ClassName, ': ', E.Message);
  end;
  Readln;
end.

If removing duplicates with higher indices don't violate your rules, just make outer loop downward. In this case removing preserves the rest.

 for I := Pred(slSrc.Count) downto 0 do
    begin
      for J := 0 to I - 1 do
 ...
         slSrc.Delete(I);