Non-uniqueness of normalized ground states for nonlinear Schr\"odinger equations on metric graphs

We establish general non-uniqueness results for normalized ground states of nonlinear Schr\"odinger equations with power nonlinearity on metric graphs. Basically, we show that, whenever in the $L^2$-subcritical regime a graph hosts ground states at every mass, for nonlinearity powers close to the $L^2$-critical exponent $p=6$ there is at least one value of the mass for which ground states are non-unique. As a consequence, we also show that, for all such graphs and nonlinearities, there exist action ground states that are not normalized ground states.