Everytime you upload a file to b2evolution, it calculates and saves a unique hash code based on the content of that file. By storing these hash codes in the database, it is possible to compare them and detect which files are identical from all the files uploaded to the user file system.
Take note that this duplicity has nothing to do with the name of the files, because the content of the files is used as basis for the comparison instead of their filenames. Conversely, files with identical names, but in different folders, are not included in the list as duplicates if their content are not exactly the same.
There is a particular case that needs to be highlighted here, because it lets you detect inconsistency between the files recorded in the database and those that are physically stored in the disk. As an example, when a file is stored in the database, and the collection in which this file belongs to has been deleted, the column Icon/Type shows the phrase Not found instead of the file icon or thumbnail, an inconsistency has been detected.