If the user pastes or wants to upload a larger query, and it takes "a while" for that data to be uploaded (likely because of crappy internet connection), the submission form appears to be frozen - i.e. there is no acknowledgement that something might be happening. Thus the user is tempted to click again, rather than patiently wait for the "job creation" to have sucessfully started.

An easy fix for that would be to make the page's visual appearance change (e.g., gray out everything, or just make the button "stay clicked") before upload begins