this my code:


var
  a,b,s,ms:string[50];
  ss,mss,k,n,i,j:integer;
begin
  readln(s);
  n:=length(s);
  mss:=0;
  for i := 2 to n div 2 do
    for j := 1 to n-i-1 do
      begin
        a:=copy(s,j,i);
        ss:=1;
        for k := j+1 to n-i+1 do
          begin
            b:=copy(s,k,i);
            if a=b then inc(ss,1);
          end;
        if mss<=ss then
        begin
          mss:=ss;
          ms:=a;
        end;
      end;
  if mss=1 then write(s) else write(ms);
end.

OR


var
  a,b,s,ms:string[50];
  ss,mss,k,n,i,j:integer;
begin
  readln(s);
  n:=length(s);
  mss:=0;
  for i := 2 to n div 2 do
    for j := 1 to n-i-1 do
      begin
        a:=copy(s,j,i);
        ss:=1;
        for k := j+1 to n-i+1 do
          begin
            b:=copy(s,k,i);
            if a=b then inc(ss,1);
          end;
        if mss<=ss then
        begin
          mss:=ss;
          ms:=a;
        end;
      end;
  write(ms);
end.

Simple: start your cycle for i from 1.