As your assets saved in the libraries grow, it is more likely you would have some identical images saved. And this could take up unnecessary spaces in your disk.
This frustration ends with Eagle's 'Duplicates notification' function, whenever you save a duplicated image/asset, Eagle will notify you and let you decide how to deal with them.

Enable the duplicate notification function

You can enable/disable the notification in the 'Preference' menu. First off, open up the preference window.

Left-click on ' General' tab and find “Notify me when importing duplicates'.

Check the 'Notify me when importing duplicates' option to enable it, and uncheck to disable.

Duplicates Notification Description

In the duplication notification window, you can see the following options:

Import
The image will be imported to Eagle and the computer will save two files.
Don't Import
The image won't be imported to Eagle.
Apply all
The option appears when you're importing multiple images. If checked, the rest of the images will be processed with the same option automatically.
Use existing file
The incoming image will not be imported, rather, the existing image will be distributed to the folder you wish to import, which means the same file will be existent in two different folders. This is made possible thanks to Eagle's unique library structure.

Notify me when images are duplicated