If two people in a list have the same email address and mail is sent to the list, only one person of those two will get mail.
If mail is being sent to a list of people and two of these people have the same email address, the mail will only send once per email (duplication stops it)
In a test case, 4 people were found in a search, two of which had the same email address. This message popped up as the mail was being sent:
But only three emails were sent. The message should state that if duplicates are found, only one email will be sent.
Communications are also being created for all four people in the list, even though one of them never got any mail.
This behaviour has been stated as by deisgn. If a user wants a different message to appear they can change the translation.