John Regan

This plugin adds a simple button to the post editor screen which allows editors to duplicate posts quickly and easily. It’s so simple that no configuration is necessary!
Features
<ul>
<li>Copies with just one click!</li>
<li>Make duplicates from the List or Edit Post view</li>
<li>No configuration needed</li>
<li>Works for all Post Types</li>
<li>Integrates seamlessly with the WordPress UI</li>
<li>Extremely lightweight (~8KB)</li>
<li>Thorough Documentation</li>
</ul>

Simple Copy Post Button

Copies/Duplicates Posts, Pages, and Custom Post Types with just one click.

ska-dev

Adds a compact group of additional buttons to the Block Editor toolbar that allow to perform common actions without using hotkeys or opening dropdown menus.
Available actions:
<ul>
<li>Insert before</li>
<li>Insert after</li>
<li>Group</li>
<li>Ungroup</li>
<li>Copy</li>
<li>Cut</li>
<li>Duplicate</li>
<li>Delete</li>
<li>Strip metadata</li>
</ul>

ska-toolbar

Adds a toolbar with quick actions to the Block Editor.

Simple Copy Post Button vs ska-toolbar