When an agency's website says "do not send multiple queries" or "only query one agent", does that mean only query one agent, period, or only query one agent until you get a rejection and then you're free to query another?
It's hard to tell, isn't it?
I know several agencies who have a central pool for queries. You send a query, they select which agent gets it and that's it. You've queried once, you're done.
Other agencies, like ours here at FPLM, doesn't do that. You query one agent, you hear back, you are free to query another. What you shouldn't do is query all of us at the same time.
So, how to tell?
The query address is a clue: if you send your query to "Queries @ Best Sellers & Arus.com" you can assume that the queries are sorted there, and it's one and done.
Another way is to pay attention to the query chatter on Twitter. @AskAgent is a good way to hear agents talk about how they receive queries.
Another way is to look at the discussion boards at AbsoluteWrite.com There's a wealth of information there (most of it correct!) about how agencies handle queries. Make sure to read the most current information first, because how agencies handle queries changes.
And in the end realize there is no such thing as the Query Police. If you query twice or three times, but you personalize the letter and don't send to everyone all at once, you're not going to end up on some urban legend blacklist. Those spots are reserved for people who query repeatedly and stupidly. The fact that you've asked the question, and are a regular blog reader precludes you from joining that list.