The email module will not allow the same email message to be sent to the same constituent id after it has already been sent.  A constituent could receive the same message more than once if the constituent email is being used on multiple records.  The system only filters out if a particular constituent has already been sent, and not if a particular email address has already been targeted.  Examples:
  1. The same email has been registered by multiple people in the household.  So they have different constituent records.  
  2. Duplicate records exist for the same constituent and their email is logged multiple times.  You can select Find Duplicates in the constituent record to see if they have the same email address, and then merge the records to prevent this problem.
  3. A donation is made on someone's behalf. This is especially prevalent with TeamRaiser and Personal Fundraising, where a constituent might be soliciting donations from others. Often people will use their email address and the donor's information, creating another record for them. In this case just opt the extra records out of email, which will allow the constituent to still receive autoresponders but they won't get multiple copies of normal emails.
  4. If none of the above apply, and you cannot find another record with the same email address, it is likely that the donor has email forwarding setup and is also registered with another email address. To the constituent it may seem like they are getting multiple copies, but in actuality some of them are forwarded. A few ways you can attempt to verify this hypothesis:
    1. Search Constituent 360 for other records with a similar name but different email address
    2. Examine the emails themselves and any personalization. Often times the email itself will contain the name of the constituent, the email address that is being targeted, etc.
    3. If you have your unsubscribe link set to go to a form (as opposed to being unsubscribed immediately by clicking the link), click on the unsubscribe links in each email. They will take you to a customized page that should verify the email address associated with the constituent record the email was sent to.