An individual email address can only be assigned to a campaign once in the RapidFunnel platform.
It is possible to add 1 contact in 2 campaigns if they have 2 different email addresses.
You do have to enter them as two separate contacts with two separate emails.
An individual email address can only be used on one campaign at a time.
These protections are in place to avoid from putting contacts into multiple campaigns, otherwise it would greatly increase the amount of email that person gets, which would increase the number of people who report our email as SPAM...which would lead to lower delivery rates for everyone on the platform.
In other words, if the contact is assigned to multiple campaigns or same campaign by multiple users, their inbox will be full of identical emails, which might become overwhelming to them. The contact might then decide to report all emails from users as spam.
Also, please see Can I add a contact if another associate has added it to the system?