Playing with friends =/= unfair.
If they are going to be jackasses and a detriment to their team then they will simply lose more than they win and go down in rank.
You are just as likely (more likely actually, slightly) to have the jackass duo land on your opposing team.
Also even if you are Duo'd you can still get another set of Duo on your team. Many times red and I have been playing with another set of 2, and with a lone solo Q player. So you are not exempt from playing with a Duo just because you are Duo'ing yourself.
BUT you should still Duo as often as you can imo. Reducing the total number of potential bad players on your team is never a bad thing.
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~Ç~ Eff_Dee
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