You may have found that you've tried to add the same contact email address to an automated campaign but the email step is not sending the email out?

A: Within each automated campaign you can only add each unique contact (with unique email address) once into the funnel.

Even if you have clicked on the "manage contacts" button and removed them part way through your campaign, you will not be able to re-add them successfully with that same email address.

This is similar in principle to our webforms being once only sign ups with each unique email address.