input type="submit", onclick handler calls this.form.submit(), and returns no value

Gecko (Firefox) certainly detects multiple submissions and cancels older ones when new ones happen. See the mPendingSubmisson member in http://hg.mozilla.org/mozilla-central/file/c4abfca219e5/content/html/content/src/nsHTMLFormElement.h and the handling of it in http://hg.mozilla.org/mozilla-central/file/c4abfca219e5/content/html/content/src/nsHTMLFormElement.cpp (e.g. in nsHTMLFormElement::Submit and nsHTMLFormElement::PostHandleEvent (the latter being what gets called from the default action stuff for submit controls).

In terms of what the spec says, it's not clear to me that the spec is necessarily sane, but it lives at http://www.whatwg.org/specs/web-apps/current-work/multipage/association-of-controls-and-forms.html#concept-form-submit and suggests that both submissions would happen, but the later might effectively cancel the earlier because of internal details of the "navigate" algorithm. I filed https://www.w3.org/Bugs/Public/show_bug.cgi?id=20580 to sort the spec out.