Depends on what era. In Europe, coats of plates didn't really appear before the 13th century and full plate armor wasn't developed until the late 14th century. Before that you mainly had people wearing chainmail and a helmet if they could get it, or gambesons (cloth armor).

At that time, weapons were still somewhat effective against armor. Spears, axes, and arrows could punch through chainmail.

When full plate armor was developed, only the very wealthy had access to it, and everyone else continued to just wear chainmail and gambesons. Fully armored knights effectively became tanks that could slash their way through all the peons.

The only realistic way the foot soldiers could stop them was to have several guys swarm an isolated knight, each grabbing a limb, and hold him down. Then they would either stab the knight through the gaps in his armor (like the eyeslot of the visor) or more likely would drag him off for ransom.

That being said, there are plenty of instances of 2 armored knights fighting each other, with them often half-swording or grappling each other to the ground and stabbing each other with daggers.

But my earlier comparison to tanks still stands. Most of the time, tanks are actually supporting infantry units, with tank v tank encounters being relatively rare. Similarly, knights spent most of their time in relatively small units killing a lot of unarmored opponents