I agree with Suctum, it's harder with a melee but not extremely so, it's certainly not a big deal at all. Even when you have a very good ping, you mostly have to predict kbs rather than react to them anyway (repulse, linebreakers, and w/e the non instant demo kb is called are exceptions because of their cast time which makes them pretty easy to double with any class, even with 150-200ms from my experience). In AoC you often need to be pro-active rather than reactive when it comes to something such as double tapping or interrupting an action to be fair (it's totally unlike some other games I've played such as Guild Wars 1 where interrupting actions by reacting quickly was a huge part of the game). The game has a certain clunkiness, a certain "lag" inherent to its mechanics (regardless of network latency or the speed of your hardware). For example look at this : https://youtu.be/nGcZaC3NCYM I see him start casting dooming presence which is VERY slow so I land a KB, and if you look closely you can see my kb hits him (combat log on the right proves it) and I even finish the cast but yet I get feared and he doesn't get kbed (and he didn't resist it or anything). That wasn't a lag spike or anything (we were both euros so mediocre latency though), it's just normal AoC gameplay. But if I had anticipated his fear I could have kbed and interrupted him (assuming he didn't see my kb coming or was too slow). Really it's the same when it comes to double tapping the quick or instant kbs, it makes little difference whether you're playing a caster or melee you have to think ahead anyway.
I probably double tap way better with my guard than my demo for that matter, simply because I'm much more comfortable with the class and can focus on analyzing my opponents' actions.