I agree that using Unix utilities is a good alternative for this problem, but note that

• For the purpose of this problem at least (and, AFAIK, always),

  sort foo.txt | uniq
  [download]

  can be replaced with a single sort command:

  sort -u foo.txt
  [download]

• By itself, sort -u (or sort ... | uniq) is not enough to solve this problem. Something like GNU's comm is also required (zsh, YMMV):

  % (comm -2 -3 sorted_new.txt sorted_exclude.txt; < dict.txt) | sort -u
  + \
  > tmp
  % mv tmp dict.txt
  [download]

  (BTW, if anyone knows how to avoid the temporary file above, I'd love to hear about it.)

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