Old Photos Keep Showing Up as New
Nearly every time I open Photos on OS X, old photos are showing up mixed in with pics I snapped just last week. My daughter is 13. Her newborn baby pictures are right there with pics from her brother's 12th birthday party from 2 weeks ago. These aren't the original pictures, out of order. They're duplicates being created for some reason.
I'll delete these duplicates, then a day or two later, others show up... Always older pics, mixed in with new ones. This propagates to all my iCloud-synced iDevices, too, of course.
Additionally, whenever I plug my iPhone 6s into my Mac, a bunch of old (sometimes years old) pics show up under iPhone 6s in Photos > Import. I'll import all 250-or so, then carefully drag them all to the trash (because they are duplicates). The next time I plug my phone in, they're back.