In Retently, a contact can have only one email address associated with it. However, in some cases, this might result in having two contact records, which are basically the same person but with different email addresses.
Currently, we don't have a merge feature, but there are two things you can do:
If one of the contacts doesn't have any feedback, then you can simply delete it and rename the other one with the needed email address.
You can manually add survey responses to the needed contact, and once you've done this, you can delete the responses from the initial contact and delete the contact altogether to ensure that they will not be surveyed in the future.