The same X coordinates can have 2 points in the SECP256K1 curve, but can there be another Y coordinate?

entirely.

If (X, Y) It satisfies the curved equation why² = X³ + 7 (for P)Then it is the same (X, -y)What modules blood Equivalent (X, PY).

This means that there are exactly two different Y coordinates for all valid X coordinates of the curve. Moreover, these points are the personal height of each other.