In Spidergap, email addresses are used to uniquely identify each participant.
If you need to reuse an email address with a new participant, then you will need to first update the email address of the old participant to avoid any issues.
In your 360° Feedback 2016 project, you have a participant called John Smith with the email address firstname.lastname@example.org
John Smith leaves the company and is replaced by Alfred Jenkins. Alfred uses the same email address email@example.com
To avoid issues you should first update the email address for John Smith. As John Smith has now left the company and won't be receiving an further emails, you can update it to a fake email address like (e.g.) firstname.lastname@example.org
Now you can add Alfred and everything will work as expected.
What happens if I don't update the email address first?
If you don't update the email address first, then when you upload Alfred's details the system may choose to use the previous name associated with the email address. This would lead to a 360 Feedback assessment being set up for John rather than Alfred.
To fix this, you would need to delete this assessment and then take the 4 steps above to create a new assessment.