How to avoid duplicates while importing photos on my Mac?

The photos.app has two ways of importing photos to the library. It will depend on the Photos > Setting in the General tab, how the photos will be imported:

• If the checkmark for "Importing: Copy items into the library" is set, Photos will copy the files into the library on import and you can safely delete them from the folder where you imported them from, but only, after you have checked, if all photos have been imported correctly and are readable.
• If the checkmark for "Importing: Copy items into the library" is not set, Photos will not copy the files you imported, but reference them in the original location and you must not move or delete the imported files.

The default is that the items will be copied. This is the recommended way. Referencing them in the original locations many draw-backs: We cannot use iCloud Photos, and there is nearly no support to reconnect items, if the reference gets broken. You may want to look at this page in the user guide for more details:

Where are the photos I imported?

On the Pros and Cons see: Disadvantages of a Referenced Library in Photos - Updated for macOS 10.15 Catalina

Added: You can easily check, if some items in your library have been imported as referenced items and must not be deleted from their original location. Create this smart album:

• File > New Smart Album
• Set the rule to "Photo is referenced".

This album will show you all referenced items. You can ask Photos to copy them into the library by selecting all items in this smart album together with the keyboard shortcut ⌘A, then use the command "File > Consolidate".

In a word: Yes.

One possible caveat: Are you running a Managed or a Referenced Library?

A Managed Library, is the default setting, and Photos copies files into the Photos  Library when Importing. The files are then stored within the Library package. Unless you've the change referenced below, then yes, once imported you can delete the copies in the Finder.

However: if it's a Referenced Library when Photos is NOT copying the files into the Photos Library when importing because you made a change at Photo -> Preferences -> General. (You unchecked the option to copy files into the Library on import) The files are then stored where ever you put them and not in the Library package. In this scenario you are responsible for the File Management. For an awful , don't uncheck that item

Hi Manash,

The easiest way of importing from a folder to the photos app is by importing it and then deleting the images in the folder after confirming the importation was successful. There are other methods of doing this but I'm not going to go into context.

Sincerely,

Gray