C++ :

#include<bits/stdc++.h>
using namespace std;
int n,a[50010],b[100010];
bool flag[100010];
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		scanf("%d",a+i);
	}
	for(int i=1;i<=n;i++){
		if(flag[a[i]]){
			cout<<b[a[i]]<<" "<<i;
			return 0;
		}
		else{
			flag[a[i]]=1;
			b[a[i]]=i;
		}
	}
}

Pascal :

var
a:array[1..50000] of longint;
n,i,j:longint;
begin
 readln(n);
 for i:=1 to n do read(a[i]);
 for i:=2 to n do
 begin
  for j:=1 to i-1 do
  if a[i]=a[j] then
  begin
   write(j,' ',i);
   exit;
  end;
 end;
end.