New iMac

Howdy -
I recently bought a new iMac and imported databases from the old iMac to the new and have set up the new iMac to be the server for the same databases. I am currently setting up my clients (iOS devices - iPad Pro and iPhone and a MacBook Pro ). My question is that the iOS devices and the MacBook Pro all have the databases currently on them. What happens when I set up the devices to now sync with the new iMac via Bonjour?? Will the databases duplicate??

If the databases were syncing with the previous Mac, they will not duplicate. They will merge since they are copies of the same database on each device.