Question: > 
> "Oh, you know Quirrell already, do you?
> 
> 
> *-- Harry Potter and the Sorcerer's Stone*, by J.K. Rowling
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> 
> 

There is a tag question that hasn’t negation thought the matrix is an affirmative. Is it ungrammatical or is it acceptable?

Answer: 
Here is how I may take this case as.

Certainly, the matrix is an affirmative. But then it's from the **asker's** perspective. The **asker is pretty sure** about the **answerer** *knowing Quirrell.* But then, the asker is **again confirming it.** 

Let us take an example. You live in Paris and I visit your home as your friend *cum* tourist. For me, it's quite obvious that you (being a citizen of Paris) **must** have visited The Eiffel Tower. I then ask you this:

> 
> *"Oh, you have already visited the Eiffel Tower, haven't you?"*
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As an example, when you say "She's reading that book, is she?", you mean that you can not believe that she's reading the book.

The speaker is expressing serious doubt about the truth of the statement by saying "Oh, you know Quirrell already, do you?". I think the speaker is probably Dumbledore and he's introducing the new master,Quirrell, to the students of Hogwarts (magic school).

These tag questions are called true-question tags or same-way tags, as I've read it somewhere.