Merging contacts is not a feature yet in Outlook for Mac...
Sorry, merging contacts is not a feature yet in Outlook for Mac unlike the Windows version which has the function already. For now, you can only manually remove them in the app. This would be a nice add-on thought and I encourage you to request this feature to our Dev team via the UserVoice channel. -> https://outlook.uservoice.com/forums/293343-outlook-for-mac
Thank you and please let me know if you have other questions or are we good to archive this already?
Chris Pike commented
It would be great to be able to set 2 contacts and right click, then merge. It is a feature that a lot of people would love to have.