HeroHunt Engage will not send candidates the same message in the same Engage flow. Any duplicated candidates in Engage flows will be ignored by design, so you don' have to worry abot adding candidates to the same Engage flow multiple times.
Tip
To look at profiles you have not seen yet, click on "add filter" and select profiles you prefer to see, like this:
Then you will see the other candidates that have not been viewed or contacted by you yet.