This will be short (but hopefully I will find more time to show entire process to search duplicated files together with some examples).

In case you are searching for duplicated files I can recommend two software which actually rocks in openSource world

1. dupegure https://github.com/arsenetar/dupeguru
   fantastic software, lots of options
   I was using that software to remove duplicates from my archive storage and it reduced the storage size by like 30-50%, in consequence eliminating the needs to buy new disks.
   
2. czkawka https://github.com/qarmin/czkawka
   this is really good software, with nice GUI
   Like dupeguru it can remove duplicates based on name, size or content (which is basically checksum calculation). The algorithm used by that software is really optimised, so you can expect improvement with comparison to other software (in checksum case all files needs to be read from the disk, so by the design it can take some time, but still wow 🙂