Additionally, in that construction 'is' links two like structures. That's why 'to' occurs twice:
[1] All you have
to do is to find them. (to-infinitive
is to-infinitive)
As for [2] (see below) the second 'to' is omitted because it's redundant. That is, 'to' has already been stated in '
to do', so the speaker/writer leaves the second 'to' out.
[2] All you have
to do is find them. (to-infinitive
is infinitive)
All the best,