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When two black holes merge in a dense star cluster, they form a new blackhole with a well-defined mass and spin. If that "second-generation" black holeremains in the cluster, it will continue to participate in dynamicalencounters, form binaries, and potentially merge again. Using a grid of 96dynamical models of dense star clusters and a cosmological model of clusterformation, we explore the production of binary black hole mergers where atleast one component of the binary was forged in a previous merger. We createfour hypothetical universes where every black hole born in the collapse of amassive star has a dimensionless Kerr spin parameter, $\chi_{\rm birth}$, of0.0, 0.1, 0.2, or 0.5. We show that if all stellar-born black holes arenon-spinning ($\chi_{\rm birth} = 0.0$), then more than 10% of merging binaryblack holes from clusters have components formed from previous mergers,accounting for more than 20% of the mergers from globular clusters detectableby LIGO/Virgo. Furthermore, nearly 7% of detectable mergers would have acomponent with a mass $\gtrsim 55M_{\odot}$, placing them clearly in the mass"gap" region where black holes cannot form from isolated collapsing stars dueto the pulsational-pair instability mechanism. On the other hand, if blackholes are born spinning, then the contribution from these second-generationmergers decreases, making up as little as 1% of all detections from globularclusters when $\chi_{\rm birth} = 0.5$. We make quantitative predictions forthe detected masses, mass ratios, and spin properties of first- andsecond-generation mergers from dense star clusters, and show how thesedistributions are highly sensitive to the birth spins of black holes.

Black Holes: The Next Generation -- Repeated Mergers in Dense Star Clusters and their Gravitational-Wave Properties