  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>10
with the DECIGO or Big Bang Observer gravitational-wave observatories, assuming
supermassive stars in the mass range 10^4-10^6 Msol. The remnant is rapidly
spinning with dimensionless spin a^*=0.9. The surrounding accretion disk
contains ~10% of the initial mass.
