This question has been asked earlier in Can I Upload the same file twice in a row?. It seems it is a bug in the dcc.Upload component.
In this question on StackOverflow there are two proposed solutions
- Modify the dcc.Upload component (JS code), ie. fix the bug in issue #816
- Use another upload component, such as the du.Upload from dash-uploader.
I am the author of the dash-uploader package and could (of course) recommend using du.Upload over the basic dcc.Upload component in almost any uploads needs (With the exception when you do not want to write anything to the harddisk of the webserver).