[johannkokos]

if (seen.find(lower) == std::end(seen)) { seen.insert(lower); return false; }

You can just return

seen.insert(lower).second

as insert function return a pair<iterator, bool>

[Nimelrian]

And today I learned something new. Been a long time since I wrote my last lines of C++ anyway.

[rndgermandude]

Bonus points for using emplace() instead of insert