If you use server-side synchronization and start seeing duplicate activities, such as tasks, inside of Outlook, that can sometimes happen if your mailbox is configured for server-side synchronization in multiple CRM organizations:

1. During deployment, if you configure server side synchronization in UAT and then again in prod, without first disabling it in the first environment. The recommendation is to re-run the mailbox test and check the box to exclusively synchronize with the production organization and disable mailbox profile in all other CRM orgs.
2. This can also happen if someone clones an org online and doesn’t disable server-side sync in the cloned org before disabling admin mode (because they didn’t listen to tip 389