The best replacement for duo is dual core.

The best alternatives to duo are rated as following:
★★★★★  dual core
★★★★★  quad core
★★★★★  duet
★★★  quad
★★  i5
★★  dual
★★  core 2 duo
★  quad processor
★  core duo
★  amd
★  i3
★  lashgrip
★  pro duo
★  pentium d
★  core 2 quad
★  google authenticator
★  duo ii
★  authy
★  toopher
★  revlon lash glue
★  ardell glue
★  amd turion
★  flip
★  magicjack plus
★  ardell eyelash glue
★  ardell
★  intel core 2 duo
★  solo queue
★  magicjack
  turion
  quad core processor
  amd x2
  duet mountain buggy
  intel core duo
  magic jack plus
  solo
  ultimo
  tilt
  dual processor
  i7
  quad processors
  memory stick pro duo
  matrix
  core duo 2
  quad intel
  effaclar k
  core i3
  leap motion
  fc twin

Your alternative for duo is not listed? Suggest!
Speaking about duo, one probably shouldn't forget about mike gordon.

Other search terms related to duo and their ratings are:
★★★★★ mike gordon
★★★ you 4shared
★★ core 2 duo
You have in mind a companion for duo and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of duo in all years. Click on graph for more.
duo
dual core
quad core