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Imagine Wizard A tries to cast fireball. Wizard B casts Counterspell as a reaction to try to nullify said fireball.
You can Counterspell a Counterspell, yes. You're allowed to take reactions on your own turn, and casting Counterspell will trigger the reaction for Counterspell. This was confirmed in a Sage Advice column:
Can you also cast a reaction spell on your turn? You sure can! Here’s a common way for it to happen: Cornelius the wizard is casting fireball on his turn, and his foe casts counterspell on him. Cornelius has counterspell prepared, so he uses his reaction to cast it and break his foe’s counterspell before it can stop fireball.
There's one important restriction to mention here. If you cast a bonus action spell,
You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.
So you can Counterspell the guy trying to Counterspell your Fireball, but you can't Counterspell the guy trying to Counterspell your Expedititous Retreat (for example).