We study the collapse of rapidly rotating supermassive stars that may have formed in the early Universe .
By self-consistently simulating the dynamics from the onset of collapse using three-dimensional general-relativistic hydrodynamics with fully dynamical spacetime evolution , we show that seed perturbations in the progenitor can lead to the formation of a system of two high-spin supermassive black holes , which inspiral and merge under the emission of powerful gravitational radiation that could be observed at redshifts z \gtrsim 10 with the DECIGO or Big Bang Observer gravitational-wave observatories , assuming supermassive stars in the mass range 10 ^ { 4 } -10 ^ { 6 } M _ { \odot } .
The remnant is rapidly spinning with dimensionless spin a ^ { * } = 0.9 .
The surrounding accretion disk contains \sim 10 \% of the initial mass .