All I can see is that seraphim and SW classes for example, have a "special" left click attack,that is somehow faster than your average attacks. Never seen 2x damages go out with one hit though. For example,when you click once on a target,the character sometimes hits twice in succession faster than just one hit,but never 2x damages with one hit. Missing something I guess,but I've spent some time trying this and the only attacks that do 2x dmg with one hit are modded Combat Arts.
In my case,with Shadow warrior and 2h sword. When I click on the target (normal attack/Left click),you do one swing and one damage number. Sometimes though,when I click once,he swings 2x times,faster than just 2 regular attacks. This got a kind of special animation but it also works without weapon mastery/double hit from gear. If that's not how double hit works for Left click,then something isn't right since I've never seen one attack dealing 2x damages. When you use modded frenzied rampage though, you can clearly see that one hit deals 2x damage.
As I said,I play the diablo 2 fallen mod,but im pretty sure it's not the issue.