Muggles' Guide to Harry Potter/Magic/Geminio

Overview
Geminio is a spell used to duplicate things.

Extended Description
The verbal component of this spell is based on the word Gemini, which is the English form of the zodiac symbol for "the twins". It is used by Hermione to duplicate the Horcrux in the Ministry of Magic, so that Umbridge will not be aware that it has been taken from her.

Analysis
Note that this spell is similar, but not identical, to the Gemino curse.

Questions

 * 1) This spell duplicates objects, so why is this spell not used much in the books? For example, in Harry Potter and the Chamber of Secrets, Harry, Ron, Hermione, Percy, Ginny, Fred and George all need a complete set of Lockhart books for Hogwarts. Seven sets of books were bought, but Mrs. Weasley presumably could have bought one set, and cast this spell to make seven complete sets for the price of one.

Re: why did Molly not use it?
The question has been asked, why don't wizards use Geminio to twin objects that they need more of? Of course, this is never answered directly in the book, but there is implication that either the copy just isn't good enough, or there is some form of magical rights management, akin to Muggle DRM or copy protection.

Taking the latter case first: We have already seen that some spells can be countered; Alohomora can be blocked, as we see in Harry Potter and the Order of the Phoenix, where Professor Umbridge has locked her door in a manner that makes it impervious to that spell, and we see in Harry Potter and the Deathly Hallows that some objects, notably the Sword of Gryffindor, are unaffected by Accio. It is to be expected that someone like Gilderoy Lockhart, concerned as he is with his own fame and profits, would have protected his books with a spell to prevent copying. Bookstores like Flourish and Blott's, of course, would be all in favour of such a spell, as they could not stay in business if their wares were copied indefinitely.

There is also the inferred "not good enough" issue. When Hermione uses the Geminio charm to create a duplicate of the locket, she says that she believes the copy will be good enough to fool Umbridge. This quite strongly suggests that even a witch as technically competent as Hermione cannot use the spell to create an identical copy. It is also mentioned that the copies produced by the similar Gemino curse are worthless. So if, as suggested, Mrs. Weasley were to use Geminio to replicate textbooks, the new textbooks might be as unusable as the end results of the failing Weasley spell-checking quill near the end of Harry Potter and the Half-Blood Prince – the text might be present but scrambled, or the books might be solid blocks that only appear to be books, for instance.