Imsanity

It has an option to search already uploaded images and resize them accordingly.

Installation:

Unpack and upload it to the /wp-content/plugins/ directory.
Activate the plugin through the 'Plugins' menu in WordPress.

What is new in this release:

• Fix for incorrectly identifying media uploads as coming from 'other' on WP 4+

What is new in version 2.3.4:

• Fix for incorrectly identifying media uploads as coming from 'other' on WP 4+

What is new in version 2.2.5:

• Fixed bug with bulk upload deleting the "large" image size in certain situations.

What is new in version 2.2.2:

• Replaced image_resize() call for WordPress prior to 3.5.

What is new in version 2.2.1:

• Removed deprecated call to image_resize() in bulk resize.

What is new in version 2.1.3:

• Fixed issue where only first 250 images could be re-sized in bulk-resize feature.

What is new in version 2.1.2:

• Really fixed bug w/ multisite network settings only working on main site.

Requirements:

• WordPress 2.9 or higher