A user has two identical phone numbers in their profile.
This can happen in Zendesk, and you can use our List Identities API to get the JSON which will show that the phone numbers are not identical (one value does not have a "+1" at the beginning). Our user interface adds a "+1" where applicable to phone numbers that do not have them.
Fortunately, this issue can be easily avoided going forward. Go to Admin > Settings > Customers and enable Validate user phone numbers. This causes phone numbers to be E.164 compliant (formatted with a "+1", when applicable) before they are saved. It would not allow a user to have two identical numbers listed in their profile.
Unfortunately, changing the setting does not affect phone numbers (user identities) that already exist in your Zendesk. You will have to delete those user identities if you don't want to see duplicate phone numbers.