  Recent advances in gravitational-wave astronomy make the direct detection of
gravitational waves from the merger of two stellar-mass compact objects a
realistic prospect. Evolutionary scenarios towards mergers of double compact
objects generally invoke common-envelope evolution which is poorly understood,
leading to large uncertainties in merger rates. We explore the alternative
scenario of massive overcontact binary (MOB) evolution, which involves two very
massive stars in a very tight binary which remain fully mixed due to their
tidally induced high spin. We use the public stellar-evolution code MESA to
systematically study this channel by means of detailed simulations. We find
that, at low metallicity, MOBs produce double-black-hole (BH+BH) systems that
will merge within a Hubble time with mass ratios close to one, in two mass
ranges, ~25...60msun and >~ 130msun, with pair instability supernovae (PISNe)
being produced in-between. Our models are also able to reproduce counterparts
of various stages in the MOB scenario in the local Universe, providing direct
support for it. We map the initial parameter space that produces BH+BH mergers,
determine the expected chirp mass distribution, merger times, Kerr parameters
and predict event rates. We typically find that for Z~<Z_sun/10, there is one
BH+BH merger for ~1000 core-collapse supernovae. The advanced LIGO (aLIGO)
detection rate is more uncertain and depends on the metallicity evolution.
Deriving upper and lower limits from a local and a global approximation for the
metallicity distribution of massive stars, we estimate aLIGO detection rates
(at design limit) of ~19-550 yr^(-1) for BH+BH mergers below the PISN gap and
of ~2.1-370 yr^(-1) above the PISN gap. Even with conservative assumptions, we
find that aLIGO should soon detect BH+BH mergers from the MOB scenario and that
these could be the dominant source for aLIGO detections.
