The best replacement for carbon copy is aqua puss.

The best alternatives to carbon copy are rated as following:
★★★★★  aqua puss
★★★★★  superduper
★★★★  time machine
★★★★  dd3
★★★★  memory man
★★★  memory toy
★★★  memory boy
★★★  blind carbon copy
★★  dd7
★★  super duper
★★  echo puss
★★  blind copy
★  dm2
★  courtesy copy
★  dd-7
★  dl4
★  copy
★  dispatch master
★  flashback
★  supa puss
★  bcc
★  boss dd7
★  boss dd6
★  ad9
★  keeley mod dd3
★  dmm
  tc electronics flashback
  malekko ekko 616
  boss dm 2
  carbon copy
  el capistan
  analogman delay
  deep blue delay

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Speaking about carbon copy, one probably shouldn't forget about business letters.

Other search terms related to carbon copy and their ratings are:
★★★★★ business letters
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Google Trends

Scale is based on the average worldwide traffic of carbon copy in all years. Click on graph for more.
carbon copy
aqua puss
superduper