I can't confirm your findings.
I have tried default CKEditor 4.5.11 full package in Firefox, IE and Chrome. Duplication happened in none of these browsers.
Looking at the image, I can see you are using customized version of the editor. Please try using default full/standard package or disable your third-party CKEditor plugins, core code customizations (if you have made any) and re-check.
If you can reproduce it in default package, please provide exact steps to do so and CKEditor configuration if some changes were needed to be made to it.
NOTE: by default the aside
tag is filtered out by ACF, so I needed to use below configuration:
var editor = CKEDITOR.replace( 'editor1', {
extraAllowedContent: 'aside(*){*}[*]'
});