How do I make sure I don't have duplicate users?

Thanks for the question Taylor. I am pleased to let you know that there are a few ways to resolve duplicate people in customer.io - manual and automatic. 

You would accomplish this by merging them. It sounds like you are interested in merging automatically. 
 

If you created your workspace using the email or ID option after September 24, 2021, we automatically recognize and merge duplicate profiles if you send an identify call with id and email, and all of the following things are true:

• The id and email in the request matched different people.
• The id and email in the request matched exactly one person each.
• The profile matched by id does not have an email
• The profile matched by email does not have an id.

In this case, when we merge people, we treat the older profile as the Primary person and the younger profile as the secondary person. I’ve included an example below: 

You can also merge people using the Track API.
The payload contains primary and secondary profile objects. As when you merge people via API, the primary profile remains after the merge. The secondary profile’s information is merged into the primary, and then the secondary profile is deleted.

The primary and secondary objects can each contain one of, and only one of, id, email, or cio_id. 
 

I recommend reviewing our Merge People Via the API documentation to learn more.