Yes, the response can only be completed once, no matter how you authenticate in the response for an individual.

You can even start it via the anonymous link with authentication, and then later complete it via the email link :)

You can use SSO for authentication, but you can also use the authenticator element in the survey flow, that would allow you to use email, user id or anything else unique to log is as yourself.

In the authenticator element, you can decide which meta data element you want to use as the authentication field.

Note that this field cannot be translated, and will just show with the name you have give it, so it might be needed to load your authentication meta data to a special field that you only use for authentication, so you can name it in a way that works across languages and makes sense to the employee.

Great to know, thank you Would the authentication only show to those who access it via the anonymous link?

For example, if email address was used for authentication for the anonymous link, would those who take the survey via their individual link also be asked for their email address? 

Hi @ChristineNZ 

You can control who should see the authentication by using branching.

In the anonymous link, you should already have this part:
&Q_EE_ANON=1

This means that you you can use this in the survey flow like this:

That way you would see the authenticate function only if you are using the anonymous link.