  In a metal with multiple Fermi pockets, the formation of s-wave
superconductivity can be conventional due to electron-phonon coupling or
unconventional due to spin fluctuations. We analyze the hexagonal diboride
CrB$_2$, which is an itinerant antiferromagnet at ambient conditions and turns
superconducting upon increasing pressure. While the high pressure behavior of
T$_c$ suggests conventional s-wave pairing, we find that spin fluctuations
promoting unconventional s-wave pairing become important in the vicinity of the
antiferromagnetic dome. As the symmetry class of the s-wave state is
independent of its underlying mechanism, we argue that CrB$_2$ is a realization
of a hybrid s-wave superconductor where unconventional and conventional s-wave
mechanisms team up to form a joint superconducting dome.
