That's a double spend, it will never confirm.
I don't understand. How can double spends happen?
They can't. That's why this will never confirm. This is an invalid transaction, because it is a FAILED attempt to double spend. The "confirmed spend" went elsewhere, and then the sender attempted to fool the recipient into accepting this invalid unconfirmed transaction.
Wasn't this solved already and it was impossible for this to happen?
It is not impossible for someone to send you a fake unconfirmed transaction that will never confirm. It is impossible for a double-spend to be confirmed. Therefore, as long as you wait for confirmation before providing anything of value in exchange, you will be ok.
Or is it normal that it can happen sometimes and it is a success that the bitcoin network can identify that this is going on and automatically reject it as a double spend?
Correct. A person can send as many transactions spending the same bitcoins as they like, but AT MOST ONLY ONE of those transactions will confirm. Once one of them confirms, all the other attempts to spend the same output will become invalid.