Just upgraded to Windows 10 Pro with Outlook 2010 using Sky as my ISP (IMAP).

Problem: Email recipients in TO box receive one copy of an email correctly, whereas the CC recipients receive the same email twice. It appears consistent . It does not happen when using webmail. we are not using any servers other than the Sky/Yahoo email servers. Sky refuse to assist

Using AVG antivirus, but we have opted not to scan outbound emails.

We have turned off all COM add-ins

We have tried changing smtp ports (both 465 & 587). Makes no difference if we use the Sky or Yahoo servers.

I have checked that the CC recipients are NOT in contact list twice.

Non of which has any affect.

I have tried googling the issue and have not got very far - hence my plea here....