I cursed Apple and Google every time I searched for an email and got a list of possible matches that was twice as long as it needed to be. But then I hit upon a simple, if imperfect, solution - just exclude the "All Mail" folder from Spotlight searches! Open Spotlight's preferences and add the folder All Mail.imapmbox to the exclusion list on the Privacy page. The folder path will be something like this:
    ~/Library/Mail/IMAP-<user>@<domain>@imap.gmail.com/[Gmail]

This makes a huge improvement but any email messages that have multiple tags in Google can still show up as duplicates in a Spotlight search. Most of my messages only have a single Google tag so it rarely annoys me. If it does get to be an annoyance I can no longer tolerate then I have another potential solution - exclude all of the Gmail mailboxes except All Mail.imapmbox. This would be more work and I would have to update the Spotlight filter every time I add a new tag. But it would ensure that every Gmail message would only be indexed once by Spotlight.