Question about the map

[Amanda]

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Movie Fact: Lupin knows the map is a map and what it does.
Movie Fact: Sirius also knows this, because he also (in the Shack) 
talks about it as if he knows exactly what it is and what it does.

Question within the movie parameters, then: Why did Lupin tell Harry 
that keeping the map was bad, reason being that the map would provide 
Black the means to get to Harry, when Black knew all the secret 
passages and entrances it shows, already?

~Amanda