I was in a game where one of the investigators started with Bind Monster, and then got Whatley's diary to get another copy. He wanted insurance against losing it.

Did I miss a rule against this or is it not covered by the rules?

Doesn't look like it is covered, but the rules do say, "When a player gains a Spell card, he takes the top card of the appropriate deck and places it in front of himself." I'd take that to mean they just take another one off the top of the deck and they have two.

what would it be good for (having two copies) ?

amikezor said:

If you happen to cast the one that says "do not draw a new spell card", you can then cast the other and then regain that spell. Other option would be to keep casting the spell until you cast the no-draw spell and then read the tome to get full cycle of spells back.

good point. thank you for the clarification. :-)