DD Thief Vs Thief and Mesmer
First off, you should be able to run this build without Infusion of Shadows. Personally I’d never leave the house without Shadow’s Embrace because even getting rid of Weakness or Cripple is a hassle without it.
As for the classes.
Generally I would have said that D/D does very well against Mesmers because you can Cloak and Dagger off the Clones and Illusions. But with the rise of PU also came the rise of Aegis, a boon that counters Cloak and Dagger rather brutally. The constant condition spam is also hard to deal with.
It’s not an easy fight and more often than not, you’re better of just walking away. The Mesmer can’t stop you from doing that. I have beaten a few PU Mesmers at times, using D/P but it’s a lengthy and boring fight and basically rests on your ability to 60%→0% the Mesmer within a few seconds.
Against other Thieves it depends. D/D has one thing going for it, and that’s strong burst. You need to be able to instant-gib people with Cloak and Dagger + Backstab, preferably from range with Infiltrator’s Signet. Otherwise you’re going to have a rough time, especially vs. D/P Blinds and P/D immortality.
D/D is a very unforgiving weapon-set as a single Aegis or Blind can ruin your day if it counters your Cloak and Dagger. And nowadays there’s a lot of these conditions flying around.
D/D works best when your targets are distracted, like in zergs or when running in a group. But for solo roaming it’s incredibly unforgiving.