cant understand steam email! (EDIT)

It means that someone tried to use the same serial number you used for one of your games.

If you grabbed a serial number off some site then that would explain it.

I keep getting emails like that as well, what does it mean?

somebody tried to activate a key you activated.

someone tried to use the same key that you activated

Are you sure it is from Steam? It looks like phishing.

Edit: guess not.

EDIT: no.

When someone tries to activate a key you activated, and clicks the "retrieve account" button, it sends out an email to the account the key is tied to.

You have nothing to fear.

You ninja'd a key, then the other ninja that tried to get the key got angry, so he clicked on retrieve account button to annoy you with the steam e-mails (or to know which account he needs to hack? idk). That happens to me very often.

you should remove your steam account name from description for security reasons

My answer is the same as everyone else's, except, specifically that happens when some "slower ninja" also tried to retrieve the account after getting the "duplicated code" message.

pedo_1337 :(