XnView: duplicate image finder

I wanted to look for duplicate images in various folders. For linux there are only a few tools which qualify for that task. XnView is one of them. XnView is provided as freeware (no ad- and spyware) for private or educational use. XnView is an efficient multimedia viewer, browser and converter. Really simple to use! Support of more than 500 image formats. The duplicate file finder feature is really nice.