Feature Request/Idea: better handling of duplicate files when adding files to upload

[ErykKul]

Overview of the Feature Request
Current behavior:

• adding file with the same name (and label) as a file that is already being added results in the new file being silently ignored
• files with different names but with the same checksum are being added without a warning

Possible improvements

• error is given (e.g., popup) when a file with the same name is already being added
• warning is given when a file with the same checksum is already being added

It might be also useful to extend the checks for identical files to the files already present in the dataset. It is currently not done. For example, when a file with a name and label already exists in the dataset, the new file gets a suffix "-1" (e.g., "test-1.json" when "test.json" is already in the dataset). A warning could be given before adding such file. Also, a warning should be given if a file with the same checksum is already present in the dataset. However, this might slow down uploads for large datasets containing thousands of files, and when uploading only few new files. Therefore, this remains out of scope for this issue.

What kind of user is the feature intended for?
(Example users roles: API User, Curator, Depositor, Guest, Superuser, Sysadmin)
Depositor

What inspired the request?
File upload feature as it is implemented now.

What existing behavior do you want changed?
Better edge case handling when uploading files.

Any brand-new behavior do you want to add to Dataverse?
File upload is a new feature that is currently being developed.

Any open or closed issues related to this feature request?
#431