Does a counterspell counter a spell that has already been copied on the stack?

When a spell or ability creates a copy of a spell on the stack, that copy is a completely independent object. Per CR 707.10, a copy of a spell is placed directly onto the stack and is not cast — it simply exists there as its own entity. This means the original spell and any copies are distinct targets.

A counterspell (like Cancel) targets one specific spell on the stack (CR 114.1). Countering the original spell does not affect any copies that were already created, and countering a copy does not affect the original. Each must be countered separately if you want to stop all of them.

Importantly, copies of spells generally cannot be responded to in the same way as the original because they are not 'cast' — they are put directly on the stack (CR 707.10). However, they can still be targeted and countered by spells or abilities that say 'counter target spell,' since they are spells on the stack (CR 111.1).

Example: Your opponent casts Lightning Bolt, then uses Reverberate to copy it, creating a copy on the stack. You cast Counterspell targeting the original Lightning Bolt. The original is countered, but the copy remains on the stack and will still resolve, dealing 3 damage.

So if you want to stop both the original and the copy, you need two separate counterspells (or a single effect that counters multiple spells). Countering one has no effect on the other.