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.

BDN Duplicate Images vs Media Library Assistant