Good point. That is why there is the image manager plugin "list uploads": https://github.com/echteinfachtv/q2a-list-uploads-page
which lets you delete unused images (blobs).
To solve this directly, it might be tricky. You could check if:
- the blob exists already (same filename and same size)
- has been created within the last 20 min or so
And then, only then, delete the former uploaded one.
Hope that helps.