Information:

• Prism version: 1.24.0
• Plugins: toolbar, copy-to-clipboard
• Environment: browser

Description
The copy-to-clipboard toolbar button has double shadows.

The toolbar css styles both button and span, not the direct child: https://github.com/PrismJS/prism/blob/master/plugins/toolbar/prism-toolbar.css#L46

Example

Screenshot of my website

Also a minimal example on jsfiddle

https://jsfiddle.net/vo6a5rxw/5/