Bad name. Quite bad name.
The problem is not that the person already knows what he's being told. People lecture each other about things they already know all the time in real life. The problem is that the conversation is unmotivated. And some writers mistake the symptom for the cause, and simply introduce, however, implausibly, a character who is ignorant, and then have the character in the know lecture him, however implausible that is.
To get the characters to lecture each other about things they know, there are all sorts of motives. All of which come with catches. An angry character may demand to know whether something they both know is true; a pompous character, particularly at a formal occasion, may deliver the speech; small children are prone to displaying their erudition to all and sundry. Then, all of these require stuff. You need to get the characters angry. You need to establish a pompous character -- preferably one who plays other functions in the story! -- and you need to keep the story from growing boring while he bores the characters. And you need plausible grounds for a child to be there and to have just learned whatever he needs to declaim on.
Regardless of the justification, it also had to be plausible dialog. The pompous character might lecture woodenly, but the angry character has to sound angry, and the child, child-like. Artful use of language is a good method of slithering the information in, even the characters are not intentionally telling people things they know, but it's especially important when they are.
Then there's the ignorant character. Not for nothing is the neophyte's arrival a popular thing, with its convincing reason why someone has to be told everything. But the dialog's got to be worked on even there. And the people have to act like they've got a newbie and not a convenient reason to info-dump.
As for the implausible newbie -- most people would keep him out of the loop. He works only with really artful arrangement of conversations about him. Plus, of course, the problem of getting him to serve a plot function.