Popup to be displayed on uploading duplicate file in the Notes and Attachment under Account Object

I have a requirement where I need to warn the user if they are uploading duplicate files against a particular Account record. The file will be uploaded, only need to show them a Warning message.
I have written a trigger on the ContentDocumentLink object which is determining the duplicates.

Can anyone tell me how shall I proceed with the warning message pop-up.
I have created a custom checkbox field on the Account object which is getting checked when the duplicate file is uploaded.Can I use this field in some way (Process builder/ any other way) to show a Warning message.

Or is there any other approach by which I can build this warning message which will act as an FYI to the user?

Thanks