Q: File management - import duplications - Photos

1 - The iCloud Photo Library does not work with a "referenced" library.  It must be a "managed" library. 

2 - If you're running a "referenced" library and move either the library or the referenced image files to another volume you will break the link between the files and the library and irrevocably damage the library.  If you import a folder into the library that has already been imported Photos will alert you with this window:

If you need additional space on your boot drive and you're running a "referenced" library you can do the following;

1 - open the library, select all photos in the library and use the File ➙ Consolidate... menu option.

This will copy the files into the library thus converting it to a "managed" library.

2 -  Move it to an external HD, formatted OS X Extended (journaled) and run it from there.

3 - After opening the library on the EHD and confirming it's working as it should delete the library and the original referenced image files from the boot drive. 

Note:  you will still need another EHD to run Time Machine from to backup both your boot drive and the EHD with your Photos Library on it.

I missed that fact.  Yes, you can move the original image files that are outside of the library to an EHD as a backup and delete them from the boot drive.

As for the duplicate detection, since you're using the iCloud Photo Library and have Optimize Storage enabled for the library I can only assume that it will still detect duplicates.  I don't use the iCPL so have no first hand experience in that.

However, I would suggest you not enable the Optimize Mac Storage option unless you find you do have storage problems on the boot drive.  That way you can instantly use any photo for printing, slideshows, export, etc. without having to download the full sized original from iCloud first.  Many experienced uses have reported that it was a big inconvenience for them. 

1 - No on several counts - iCloud ins not a backup service, it is a syncing service and any change made on any connected device is made on iCloud.com  and all devices - you must have a local backup also

And the size of the local copies depends on teh settings you choose - with down all originals to this computer all photos are the original with no reduction - with use optimized photos then the photos may be smaller if space is needed on the device

And the managed option is really the only viable option - referenced libraries are strongly not recommended with Photos - Disadvantages of a Referenced Library in Photos

2 - Photos does look for duplicates during import and when it finds one it gives you the option of importing the duplicate or not - as to importing gthe Pictures folder - not a good idea since by default your Photos library is in the pictures folder

LN

I do not know the rules used to identify potential duplicates.
But recently I imported a lot of duplicates, accidentally:

I had about 300 photos already imported to Photos 2.0.

File names being like this: DSCN2915.JPG

Then I found some more photos from the same event, on a backup.

As I was not sure if they are already in Photos, I imported them.

Photos told me they are all new. And I did not check, just say OK please import.

Later I discovered that they are all duplicate.

For some reason I do not know, this photos ended in .jpg and not .JPG (all caps).

It seems that Photos thought the photos are new, because of jpg instead of JPG.

For some of those duplicates, the photo in my Photos database was already rotated, while the photos from the later import where still to-be-rotated. Maybe this also made Photos think they are not the same.

So I have re-check manually the photos of this event to remove the not recognized duplicates.

Does anyone know the rules used by Photos for checking duplicates? (edit: sorry, I should open a separate post for this)

I know iCloud is not a backup service.

then why did you make this statment

The iPhotos library is doubly backed up, both on an external local drive and also in a cloud backup service.

and you never ask about two copies inside Photos adn outside of Photos - yoou ask about duplicate detection when importing into Photos

This topic seems to be related to how iCloud articulates with Photos on the local disk, but I don't see how that impacts keeping a copy of the images in both the Photos library and also in subfolders of the Pictures folder.

which is why the =answer relates to duplicates when importing

I am getting the impression that Photos is for the casual user with a few hundred photos who doesn't want to deal with issues like image management of a large group of photographs.

What ever - you seem to prefer mis-statements and aruging and do not really want help so I am out of here

BTW I have a 20,000 photo library and an 80,000 photo library - Photos works great on both - several other expert users here have over 50,000 photo libraries

Have a nice day

LN

I missed the Carbonite sentence.  If you're backing up the Photos library package to Carbonite that will not return you a working library if you try to restore it.  The volume format used is not one that supports the complex ecosystem of files, permissions, links, etc. that make up a Photos Library.  For other files it would be OK. 

If you don't have a local backup of the library on a volume formatted OS X Extended (journaled) you don't have a backup.

Then you're covered.