ValidationError: email_id_2 field cannot be set as unique in tabContact, as there are non-unique existing values

Trying to update to V8 but i amgetting below error:

frappe.exceptions.ValidationError: email_id_2 field cannot be set as unique in tabContact, as there are non-unique existing values

I have customized field as below:

Contact-email_id2
Lead-email_id2

What we have to do to process updating our erp?
Thanks.

Below is the screenshot as additional Reference.