William P. Davis

This plugin allows you to duplicate images you have in the media library and attach them to a new post, rather than having to create a new copy of the image and upload it to WordPress again.
It works by copying the post and its metadata. It does not actually copy the image. Any changes you make to the new copy of the attachment — updating the caption or cropping, for example, will only be applied to the new attachment, not to the original.
This is useful if you query posts for its attachments to display photos, or if you make use of galleries and have old images you might want to include, among other things.
To use, go to the Library tab in a post. You should see an ‘Attach’ link.

BDN Duplicate Images

Allows you to duplicate images in the media library and attach them to the current post.

Milan Dinić

Matt Mullenweg

<a href="http://blog.milandinic.com/wordpress/plugins/cache-images/" rel="nofollow ugc">Plugin homepage</a> | <a href="http://blog.milandinic.com/" rel="nofollow ugc">Plugin author</a>
Cache Images is a plugin that gives users option to sideload images that are hosted on other domains to their own site. Sideloaded images are added to WordPress media library so you can use all tools related to images that you can use with images uploaded through WordPress. Image will be added as an attachment of first post where it is found, and every post where original URL is occurring will be updated with new URL. User can select from which domains to sideload images, including Blogger’s domains.
It uses AJAX so it means you can sideload large number of images even on slow servers. (AJAX functions are made by fork of code from plugin <a href="https://wordpress.org/extend/plugins/ajax-thumbnail-rebuild/" rel="ugc">AJAX Thumbnail Rebuild</a>)
This plugin is fully internationalized. You can find .pot file in languages folder where you should place your translation. Current translations are Serbian, Spanish, and Persian. Please send your translation by <a href="http://blog.milandinic.com/contact/" rel="nofollow ugc">contacting author</a> so that it can be included it in next releases.
Read more information about usage on <a href="http://blog.milandinic.com/wordpress/plugins/cache-images/" rel="nofollow ugc">author’s site</a>.

Cache Images

Goes through your posts and gives you the option to cache all hotlinked images from a domain locally in your upload folder

BDN Duplicate Images vs Cache Images