google-photos
I've installed Google Photos on my iPhone with the auto-backup feature.
I also have the possibility to install the backup feature on my Mac.
The problem is that iCloud syncs my iPhone pictures with my Mac. So I am afraid if I install Google Photos on my mac I'll have thousands of duplicates.
Is Google Photos smart enough to detect duplicates?
You should be able to go to:
https://www.google.com/settings/storage?hl=en
Then hover over the pie chart and it will bring up a window showing your detailed usage. Google Photos should be in the list, and that should be your total amount used by your photo storage in that folder.
You should check if there is enough space left on your drive, since Google drive changes the settings to high quality when the space in below 20%.
If that's not the case, this might be due to your connect speed, the drive shows a smaller version if it detects slow speed, however you can open image only in new tab, which would load the original.
Best Answer
Short answer
Google Photos appear that has a duplicate prevention feature that avoid to create duplicates on uploading time but it's not documented, so proceed carefully.
Alternatives
Use Picasa instead of Google Photos to backup your Mac photos.
References
Google Photos Help Forum