Yijinglin try piping in a timestamp to the body of your email. That should allow you duplicate distribution. See my quotes from this post:

If you are going to use the Qualtrics email distribution you may have trouble with it flagging those same contacts month over month as duplicates. The email support pagedefines duplicate emails as "emails with the exact same message and recipient as another email that went out." By piping a timestamp or date into the body of your email, you should be able to prevent emails being flagged as duplicates.

bstrahin . Thanks for the response! The issue we had is that when the respondent finished the survey last month, he is not able to respond to the same survey this month. It shows that the survey has been finished. We have to use authenticator to verify user's credential to track the user.

Yijinglin by piping in the timestamp this is generating custom individual links from one email blast to the other (e.g. month to month). In the string where you see Q_DL= that chunk changes allowing the person to take the survey again without getting the error message you are referring to.
You can still know their embedded data saved in the contact list by loading that in the survey flow:

• Add new element


• Embedded data


• Add From Contacts


• Navigate to your contact list (you see I have several things in my test list)

bstrahin We use Okta to authenticate the user. The scenario you described. Can you prevent the user to take the survey multiple times within a month?

Yijinglin when you say you email the user each month I am assuming you are using the Qualtrics distribution tool with individual links (I'm unfamiliar with Okta). Using the Qualtrics distributor as I described above, multiple submissions cannot be made for the same link/person in one month.
The invitation email will have a unique individual link each month. That link will store its own history. The survey remains open for completion until submitted (or expires, set that in the distribution). The Qualtrics distributor can be used to send reminders to non-completers. Then the link doesn't change from invitation to reminder, so even if the person has an invite and reminder email, the link is the same and only allows them to submit it once.
The important feature of Qualtrics is that a link can only be completed once and is in accessible after completion. The respondent gets access the survey the next month because their individual link has changed.
I assume Okta is a form of SSO authentication so it is a step in the survey flow that is only to confirm identity and not store information meaning it shouldn't impact the way the distribution tool in Qualtrics works to achieve what you want.