Apple Mail app search filter brings the same email hundred times
When you create an email Mail.app saves a draft. When you type a few more letters Mail.app deletes the old draft and saves a new one.

If you have "store draft messages on the server" selected (Preferences -> Accounts -> (select account) -> Mailbox behaviours) then every time you make an edit Mail.app sends it to Google then deletes any old drafts. Unfortunately, Google doesn't really delete the old drafts. Instead it moves them to the Bin folder and when you search in Mail.app it finds these hundreds of old drafts.

To stop this occurring on future drafts uncheck "store draft messages on the server" for all my Gmail accounts.

To clear up your old drafts log into to Gmail directly and:
* Go to the All Mail folder and manually select and delete all the old drafts
* Go to the Bin folder and select "Empty Bin"