When viewing email statistics under the 'Statistics' Tab, you will see both Opens and Unique opens.
(This also applies to 'Clicks" and "Unique Clicks")
"Opens" is the number of times your email campaign has been recorded opened. An email can be opened multiple times, and it will be recorded multiple times.
- "Unique Opens" is the number of unique recipients who opened the email.
Contact A open his email 3 times, Contact B open her email 2 times, Contact C open her email 5 times.
Opens = 10
Unique Opens = 3