Shorter still:

    sort -u < list_of_numbers

And if you're using cat because it keeps the filename out of the way when editing the pipeline, then just put the redirect before the command instead, so instead of e.g.

  cat file | grep pattern | sort -u

you can write

  < file grep pattern | sort -u

and the filename is out of the way compared to

  grep pattern file | sort -u

This is not the same. For sequence [5,5,4,3,3,2,1,1] "sort -u" returns [1,2,3,4,5], while "sort | uniq -u" returns [2,4].

Huh, I didn't know that! Thanks.