I had a problem that was driving me crazy--I had too many duplicate contacts in Outlook, and Outlook doesn't give me any way to easily merge or deduplicate contacts. So I made a PowerApp.
This app allows you to pick two contacts from your Outlook contacts, select which values you wish to keep from each record, then merge to the main contact record and delete the duplicate.
Note I started with the Contact template, which is decent but doesn't include any merge functionality. The super dooper merge feature is my addition. Also: I am not responsible for any contacts that get deleted from your outlook in this process. Fields included in the merge process may not include every field you use, feel free to extend to include those fields.