How to compile a list of unique words in a file?

On a previous *nix system, I had a little shell script named 'unique'
(contents shown below the line) that would compile a list of all the
unique words in a named text file. But when I tried to use it on my
current Linux system today, I discovered that I do not have 'deroff'
which is required for 'unique' to work.

Is there some other way to compile such a list of unique words;
or is 'deroff' something that could be downloaded from somewhere?
I googled for it but couldn't find anything that I could download
among the 2,000+ hits I got.

Also, even more to the point ... is there some kind of Linux utility
that will compile an index of words and their corresponding page
references within a given text file?

joe

-----------------------------------------------------------------
: unique -- to find and list all the unique words in a text file
# syntax: unique filename (or pipe to a new file)

deroff -w $1 | sort -uf > word.list

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