Steps to reproduce:

• click the Upload Sketch button
• click it again

Expected:
When you click the upload button the first time, and it starts uploading, the upload button should switch to a "abort upload" button (suggested icon: an X). So by clicking it a second time, it should abort the upload, the button would revert to an upload button, and you could restart the upload by clicking a third time.
As a second-best option, it could be almost acceptable that there's no such thing as an "abort upload" button, but when you click the upload again while a sketch is already uploading, the current upload should be aborted before a new compile-upload cycle starts.

Observed:
When you click the upload button the second time, a new compile-and-upload cycle starts, while the previous upload still goes on (!!!!), leading to all sorts of pathetic errors and unpredictable, disastrous behavior.