Fast Duplicates Remover

Fast Duplicates Remover is capable of removing all duplicate lines from a text file. This is a feather-light application that doesn't require setup, and it contains only a couple of options.

Delete duplicate lines from .txt docs

As far as the interface is concerned, it opts for a standard window that displays all settings available. The process is quite simple: select the text document you want to scan for duplicate lines, specify the name and directory of the new file for saving the changed document, and click a button to let Fast Duplicates Remover handle the rest.
By default, the utility is set to replace the original input file instead of creating a new one. However, you might consider deselecting it, unless you're making a backup of the original file.

How it actually works

The tool carries out tasks quickly, depending on the size of the document. It has minimal impact on computer performance, shows a notification message at the end of the job, reveals the total words and removed duplicates, and automatically overwrites files with the same names.
It keeps one occurrence of each line and gets rid of the rest. On the other hand, the developer doesn't mention that it automatically sorts the lines when creating the new document, in alphabetical order (in this order: blank space, numbers, letters).
Moreover, it doesn't have an option for showing the lines removed, and it's not clear about which file types it supports. It worked smoothly with TXT docs and we tried it with other text-oriented file types too, but the text was scrambled post-processing.

Conclusion

Fast Duplicates Remover is a pretty old application that doesn't seem to be in development anymore. Although it's speedy and serves its purpose, its mode of operation is confusing when running it for the first time.