How do you avoid duplication of work

It’s important to remember job sharing means double the  brainpower, not twice the admin.

The job role itself (Project Leader, HR Director, Marketing Manager, etc.), is still a singular facet, just run by two people.

Your job sharers will often use a shared inbox tied to the role. Likewise, most modern communication channels (Email, Slack Bios, etc.) have the functionality to include info about  which person works on which day to make it easy for the business to both understand and manage