Your tester does not know what he talks about. If you specify the same address multiple times, it will be send multiple times. It does not care in which of the destination fields ("To", "Cc", "Bcc") the address is specified. The SMTP server will parse all fields and forward the message to each listed recipient.
Mail clients will just show the content of the "To" and "Cc" destination fields. An address listed in the "To" field only will not be shown in the "Cc" field of the client.
Your tester should understand the purpose of those fields:
"To" is for the actual recipient(s) which might answer.
"Cc" is for additional recipients to get them informed but not expected to answer.
"Bcc" is like "Cc" but the field is removed from the mail (or modified) upon transmission.
See also section 3.6.3. Destination address fields
of RFC 2822