Ninja boy isn't telling you that he joined sooner than you did, just that he's welcoming you to the forums even though he hasn't been here for too long himself.

And if you didn't know, double or triple-posting is when you post multiple replies in a row in a thread.

Usually, this is only considered "bad manners" if you post twice in a thread within minutes of each other (in example, if you forgot to add something and you add a post saying just "Oh, I forgot to say, Hikki rocks!). If you wanted to add that kind of comment, you could easily use the edit button at the bottom right corner of the post and add it into the existing reply.

I don't know about Q, but if you double post days apart, I have no problems with it.