this can be done by negating a selection if the user is selecting 1 file at a time to queue up but if a user is selecting multiple files at a time and one fo those files happens to be in the queue already the only way to prevent that duplicate from showing up in the queue is to cancel the entire selection since there is no way to cancel only the selection of the duplicate file or files if there is more than one. 

one potential solution is to create a custom queue that does not display duplicate files and then prevents the saving of those files on the back end however the problem to that approach would be that the user then has to wait patiently while the duplicate files are uploaded regardless of whether or not they are actually saved after upload.

How can we prevent  the upload of duplicate files in a clean and intuitive way that allows the selection of multiple files and only cancels the duplicate's selection and allows the rest to be queued up?