There is a hidden default for Mail, which I do not think has been mentioned here. You can avoid receiving duplicate Messages (e.g. from mailing lists) by opening a Terminal and typing:
To disable the feature, repeat the command with a '0' at the end instead of a '1'.
defaults write com.apple.mail UniqueMessagesBeforeRouting 1Before receiving the whole message, the message ID (a mail header) will be checked. If there's already a mail with this message ID, it will be discarded.
To disable the feature, repeat the command with a '0' at the end instead of a '1'.
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