Using iCloud Photo Library and Google Photos Simultaneously
I have an iCloud library of about 25000 photos and videos, running on multiple iOS devices in "optimized" mode. Each device had to go through the complete backup process of uploading every image and video. Google Photos' iOS app will upload images first, then videos.

Which creates some problems on the iOS devices: With the "optimized" setting iOS only keeps the most recent or recently viewed assets in full resolution. Since google photos requests 100% of the full-size assets the iOS device, in my scenario, inevitably runs low on space for as long as google's upload process takes. Very. Annoying.

But fortunately I don't get any duplicates in google photos. I wish google would figure out a way to intelligently avoid having to upload what's already there, as this process, with a library of +150GB takes weeks.