We need Nintex to find a way to remove duplicates (ignoring case) for email addresses in the to, cc and bcc fields of send an email action!
If not this, then allow the ability to remove duplicates in a collection with a checkbox or ignoring case. This becomes extremely challenging when sending emails to groups of members in a SPO mixed with other outputs of email addresses that may have the same email address but proper casing vs all lower casing!