Everything to appear once.
Double Images and Buttons appeared while Editing an Image and Double Buttons while editing other types of media.
Happens on all sites regardless of the Plan.
Chrome 61.0.3163.100
While editing an Image:

While editing a Video:

@stephanethomas This looks like another side effect of https://github.com/Automattic/wp-calypso/pull/18194.
As a workaround, I loaded the post editor first (as mentioned in https://github.com/Automattic/wp-calypso/issues/18460) and then followed the steps in this report and the modal works as expected then.
It is, good job Rachel. I've just checked and https://github.com/Automattic/wp-calypso/pull/18477 will fix that issue as well.
Thanks @otpidusprime for reporting this issue! This is now fixed.
Most helpful comment
@stephanethomas This looks like another side effect of https://github.com/Automattic/wp-calypso/pull/18194.
As a workaround, I loaded the post editor first (as mentioned in https://github.com/Automattic/wp-calypso/issues/18460) and then followed the steps in this report and the modal works as expected then.