Problem/Motivation
Webform adds a $form['#method'] ('get') in lowercase, and this then is not matched by Drupal.behaviors.formSingleSubmit 'form:not([method~="GET"])' javascript. Thus cannot be submitted twice.
The lowercase method name could also be set via form alterations etc.
Proposed resolution
The single submit form selector should be case insensitive.
Remaining tasks
Needs JS tests.
User interface changes
None.
API changes
None.
Data model changes
None.