Salesforce Error: Lookups on activities must have a unique domain. You cannot have multiple relationships to the same object.

This issue happens when there’s already an existing lookup relationship field on an object and then the user creates a second lookup relationship on that same object.

You can only have one lookup per object on the Activity object. Because of the complexity of the Activity object, you're only allowed one lookup to a specific type of object (for example, User) for custom Activity lookup fields. 

This is a system limitation.


Workaround:

You can delete the existing field on the Activity object. While in Lightning Experience, do the following steps: 

1. Go to Setup > Object Manager > Activity > Fields & Relationships
2. Click the drop-down arrow on the specific custom field
3. Click Delete.

Note: Make sure to hard delete the custom field by switching to Salesforce Classic after performing the steps above. Failure to do so will still result in a validation error.

4. Switch to Salesforce Classic
5. Go to Setup > Customize > Activities > Activity Custom Fields > Deleted Fields
6. Click Erase next to the field you wish to be permanently deleted.