개인 메시지에 같은 사용자를 두 번 이상 추가할 수 없어야 합니다

[Macaw]

You’re allowed to add the same user more than once while composing a message:

Which is a bit odd, don’t you think?

[TheBestPessimist]

In this particular case, spamming @ codinghorror sounds fine to me

[sam (Sam Saffron)]

It is indeed odd, but not expecting anything bad to happen, the server handles this type of abuse gracefully.