How to remove duplicate words from a plain text file using linux command

Assuming that the words are one per line, and the file is already sorted:

uniq filename

If the file's not sorted:

sort filename | uniq

If they're not one per line, and you don't mind them being one per line:

tr -s [:space:] \\n < filename | sort | uniq

That doesn't remove punctuation, though, so maybe you want:

tr -s [:space:][:punct:] \\n < filename | sort | uniq

But that removes the hyphen from hyphenated words. "man tr" for more options.

ruby -pi.bak -e '$_.split(",").uniq.join(",")' filename ?

I'll admit the two kinds of quotations are ugly.