Did your ledger show a second transaction or a second output (should have been named 'Output #2' on the ledger i believe) ?
(1)
If you were shown two outputs in one transaction, then you are using an outdated version.
Your coins would be safe in this case, but i would highly suggest updating the firmware of your nano s.
A guide can be found on the official ledger site.
(2)
One plausible explanation in case of two transactions (without initiating the second one on your pc) might be malware.
If an attacker has full access to your system he definetely could do that. The screen/2FA of the nano s is the last line of defence in this case. You might have opened the door there
![Undecided](https://bitcointalk.org/Smileys/default/undecided.gif)
I am assuming it is case (1) ? I have never heard of an attack regarding hardware wallets similar to (2). But this does not exclude the possibility.