If I have Solosis and Duosion on my bench, does Duosion's "Double Link" attack do +90 more damage (30+60)?
Yes, the attack does additional damage for both Solosis and Duosion if both of them are on your bench. But you do not get to add the benefit multiple times if there are multiples of the same name; for example, if you have two Solosis, you don't get to add the +30 twice.
Source: Fates Collide FAQ (2016-05-05), TPCi Rules Team (2016-05-05)