Best practice for syncing photos without iCloud Photos to avoid duplicates?

[wlow3]

I have 3 devices: an iMac, an iPad and an iPhone. I want to be able to sync the photos taken on my iPhone, say, and also have those appear on my iPad and iMac - same with the photos that are taken on the iPad for the other two devices. I've noticed that after having done repeated backups of my iOS devices to my iMac, I'm getting duplicates. The duplicates on my iPhone cannot be deleted from my iPhone - the option isn't there.

Is this because the originals are still in the camera roll and when I sync, the iMac puts them back on the iPhone but in a different folder (and makes it so one cannot delete these without first deleting them from the iMac)?

I have a lot of photos and I don't want to pay for iCloud Photos storage.

Is there a way to sync the photos other than using iCloud so that all devices only have one copy of each? Should I just AirDrop between devices and not "sync"?

Thanks.