What's the best way to answer the same question that's been copied into multiple categories?

That's kind of a tough call, and maybe more a matter of personal judgment than an absolute adherence to or violation of the rules.

Sometimes a question really IS appropriate for more than one category. If an identical or similar question is concurrently posted in 2 or 3 APPROPRIATE categories, I'd say answer one or all if you wish to.

How (or whether) to answer is up to you. There's nothing in the rules against copy/pasting your own answer as long as the information you give is relevant. I don't see the point in providing a link back to the asker's other question ... and answering a question ONLY with a link is just begging to be reported for spamming.

However ... if a question is appearing in 5 or 10 (or more?) categories at once, it would probably be considered spamming and SHOULD be reported. You could report one question and provide links to the others. Reporting each question individually would likely be ineffective since 'spam' implies multiple instances of the same posted content, and a single example does not provide evidence of that.

There is also a thread on the Y!A suggestion board dedicated to identifying and removing spam, but that is primarily for spambot-type accounts (usually advertising), and what you're describing doesn't seem to be of that nature.

Limit your words used to describe what you are trying to find. Also an example for google would be

at follows. " quotation marks around the topic you want info. about" then a + sign added then write

the rest of what you are looking for. Good luck.