Welcome to ENWorld
Let me say, frankly,
screw that. DMs "shake things up"
all the time; it's practically part of the job description. Nothing malicious about it. Players want a fun challenge, and you want to give them one.
It's great if your player likes to plan for encounters. But D&D is not a MMO. If he throws a fit, you may need to remind him of that. You are the DM, after all. The great thing about D&D is its ability to accommodate wild ideas on both sides of the DM screen in real-time with human creativity.
Being sportsmanlike does
not mean you need to be predictable.
Why do you think that is? I'm not being facetious. What do you observe at the table that you think leads to them stomping over "hard" encounters?
For example, is it because you're only running 1-2 fights per day so the PCs can expend most of their resources in just 1-2 fights? If that's the case - for example, in an
Out of the Abyss style Underdark exploration - you might consider changing how long rests work in your game.
Your party seems geared for fighting. Also, I notice you seem to use "encounters" to be synonymous with "fights." That leads me to make some assumptions about your style of game... However, I'm still going to offer this advice:
Give them fun stuff to explore! Interesting NPCs to dialogue with! Ethical dilemmas to face! Provide encounters where combat is clearly not the best option. Change things up.
Also: Your party is conspicuously missing a rogue or a utility-based wizard who would do things like disarm traps, unlock doors, scout ahead, etc. Just saying.
Flank your enemy in a passageway. Cast spells to prepare. Box them in.
It's a sound tactical approach...if the duergar indeed know about the PCs and have time to cast buff spells in advance...if the duergar win initiative (or get surprise) and manage to lay down their
silence spell in time...if the PCs don't become aware of the duergar and organize their own counter-ambush...if the PCs don't have a way to burrow through stone...if the front-line PCs don't push back the duergar...if...if...if...
The problem I see is that it doesn't make for a terribly interesting encounter.
There's precious few interesting choices for the PCs to make.
The "Search and Destroy" duergar party isn't anywhere near overwhelming enough to inspire suspense/dread.
There's no creative gimmick which leaps off the page and would make me as a player go "wow! that's cool!"
There's no investment for me as a player beyond the rote "survive, kill all monsters, maybe take a hostage to interrogate."
If you want further suggestions – it's very possible you don't and are content with this scenario – feel free to ask.