Suppress Duplicates is a configurable setting that is enabled by default for all Campaign Email Actions and works to prevent sending the same email to the SAME email address multiple times within a 24-hour period.
If this setting IS enabled (box checked), the Email Action will only be allowed to send the same email template to different prospect records with the same matching email address once within a 24-hour period. If you have duplicate records with the same email address passing through the same Email Action, evaluating to receive the same email template (based on template ID), within the same day (24 hours), only ONE of the matching records will be arbitrarily sent the email and the other(s) will not be sent an email.
Note: Emails sent using the Test Action and Send Proof features are not counted as being sent during the 24-hour period. Only emails sent normally through a campaign are taken into consideration.
The matching prospect records that are not sent the email are considered Suppressed and are counted in the Suppressed statistic that is available for the corresponding Email Action.
For example, John Smith and Jane Doe share the same email address - firstname.lastname@example.org. They are both included in a campaign and pass through the Email Action which has the 'Suppress Duplicates' setting enabled. An email is sent to John but not Jane. John will increment the 'Sent' stat while Jane will increment the 'Suppressed' stat because Jane's prospect record is considered the "duplicate" record.
Note that suppressed records will proceed to the next actions in the campaign flow and will be evaluated normally according to the campaign design.
If this setting IS NOT enabled (box unchecked), the Email Action will be allowed to send the same email template to different prospect records with the same matching email address an unlimited number of times which is NOT recommended.