A user has two of the same phone numbers in Zendesk.
Yes, this can happen. It's not ideal, but it's expected behavior. You can use our List Identities API to get the JSON. There, 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, like you are experiencing.
Unfortunately, changing the setting does not affect phone numbers (user identities) that already exist in your Zendesk. You'll have to delete those user identities if you don't want to see duplicate phone numbers.
I hope this makes sense. Please add a comment if you have any questions.