The “check if contacts are in other campaigns” setting works at the campaign level.
When enabled, it checks whether a contact that qualifies for the current campaign has already been delivered through another campaign.
If that contact has already been delivered elsewhere (depending on your re-identification and exclusion settings), the system can exclude them from being delivered again in the current campaign.
Important clarifications:
This does not merge campaigns.
It does not delete the contact from other campaigns.
It does not retroactively change past deliveries.
It only affects delivery going forward.
It works alongside your campaign-level re-identification rules.