Sending to subscribers that belong to multiple groups

If you send the campaign out to these multiple groups at once, the system will weed out all duplicates and all subscribers will receive 1 email each.
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Please note that this isn't the case if you send a campaign to one list, and then send the campaign to another list at a different time, since this is not the exact same campaign.
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Another important point, on excluding groups in a campaign, is if a subscriber is part of both the included and the excluded group. The exclusion will prevail, and that subscriber will not receive the email.