Maintaining Unique Keys When Offline

If you have a "Orders" table that is being sync'd to subscribers that are ocassionaly offline, and the subscribers add rows to their local Orders table. When they go online to sync with the published "Orders" table, how do you handle keeping the "OrderId" field unique?

Example:
Both salespeople sync the following data down:
OrderId Desc
1 Order 1
2 Test Order



Both salespeople go offline and add orders
Salesperson 1 adds:
OrderId Desc
3 Joes Order

Salesperson 2 adds:
OrderId Desc
3 Kathys Order


Now, when they go back online, they both will sync their orders up to the main database and they both have the OrderId of 3.