Exclusion rules skip leads who were already touched so the same person is not enrolled again. The three toggles live on campaign Settings, and enroll preview shows how many will be skipped.
Turn the three toggles on
Open the campaign, then Settings → Exclusions. The labels are exact.
Open the campaign and stay on Settings.
Find Exclusions.
Turn on Exclude leads contacted in other campaigns to skip leads with an active or paused enrollment in another campaign.
Turn on Exclude leads messaged by other senders to skip leads another sender already messaged.
Turn on Exclude leads contacted by same sender in other campaigns to skip leads this campaign's senders already touched in another campaign.
Save the Settings tab.
You can also pick lists to skip. The page says "Leads on any selected list will be skipped at enroll time." After launch, senders, schedule, and exclusions stay locked until you select Edit campaign.
Check skips before you enroll
Enroll preview counts the list against these rules before you enroll.
Open enroll on that campaign.
Read the line "{total} leads in {list}: {willEnroll} will enroll, {skipped} will be skipped."
If the skip count is a surprise, change the toggles (after Edit campaign if the campaign is Active), then preview again.
InMail cannot launch as a step. These toggles still skip people who were already contacted.