I have a laser with a 14 kW power output concentrated into a 1 cm diameter circle.
Power per unit area
An industrial laser cutter for steel operates in the 500 - 1.5kW range. You have a lot more power. OTOH you have to go through more material. OT3H medieval steel stinks compared to modern steel.
14 kW puts it in the power range of lasers tested to shoot down missiles, clearly a class 4 safety hazard that will at minimum burn skin.
However, industrial lasers are usually focused to less than a millimeter, in part because the target is also millimeters away. The real question isn't about raw power, it's about intensity: power per unit area.
Lasers lose coherence as they travel through the atmosphere. The further away the target is, the larger the area it falls on. You don't say how far away your laser has a 1 cm diameter circle, but I'll guess it's within the few dozen meters of a dueling ring.
Doesn't matter, it's enough to calculate the intensity of the laser, but know that intensity changes inversely with the range.
A circle has a surface area of $\pi d^2/4$. So your 1 cm circle has a surface area of $0.79 cm^2$ or $7.9 x 10^{-5} m^2$. That works out to an intensity of $18 kW/cm^2$. That's roughly the power output of a your typical car falling on an area smaller than your fingernail.
Wavelength
The missing piece here is the laser's wavelength. Different wavelengths will transfer energy to different materials at different rates. Military and cutting lasers are in the micrometer (μm) range, so go with that.
Has a nice little reflective plate armor, shield, and sword
Medieval steel will not reflect a laser of this power, I don't care how well polished it is. Furthermore, something which is polished to reflect visible light probably won't reflect light in the micrometer range, so it's irrelevant.
[Their] helmet is protected against lasers, completely (a wizard did it)
Don't aim at their head.
Holding it on target
This is the real problem. Developing lasers to shoot down missiles isn't just the laser power, but also holding it on target. That's done with computerized stabilization. You've just got your hand trying to hold on a target the size of your fingernail while that target is trying to kill you.
Your hand will shake, the knight will move, and the power of the laser will be spread out through a much, much larger area over a much, much larger period of time greatly defusing its effect.
At your power levels it might still cause burns to exposed skin, or even ignite exposed clothing on fire, but a knight will be used to pain and will close the distance and cut you down.
Test the laser beforehand
You have a night to prepare for the knight. Get some armor plate, wave the laser around on it to simulate the knight moving around, and see what happens. If that cuts through, you're good. Confidently stride into the ring and slice him up.
If it doesn't, go find a cow and do the same thing. If it burns the cow, you're in good shape. There's enough exposed skin, clothing, and leather (ie. cowhide) on your standard knight's armor that you can wound him badly.
Fair fights are for suckers
...but you should probably get rid of that armor before the battle even begins. Armor is custom fitted and cannot easily be replaced at a moment's notice.
Before the battle, while their armor and equipment is hanging on a rack and an easy target (armor is heavy and the less time spent wearing it before a duel the better), use the laser from a distance to covertly ruin the armor. You can steady your aim on a surface, and the armor isn't moving. Burn through the leather straps which hold the armor together and the whole thing will fall apart.
The knight, now lacking armor, will have to face you without that advantage. A much easier target for your laser. If they try to delay to get the armor prepared, boast that you were ready to face a fully armored knight with nothing but the clothes on your back.
