Set<String> Accountids = new Set<String>();
for (Account a: [Select Id from Account limit 5])
    Accountids.add(a.Id);
system.debug('The size of the set is'+accountids.size());
for (Account a: [Select Id from Account limit 5])
    Accountids.add(a.Id);
system.debug('The size of the set is'+accountids.size());