  We study an active-region dextral filament which was composed of two branches
separated in height by about 13 Mm. This "double-decker" configuration
sustained for days before the upper branch erupted with a GOES-class M1.0 flare
on 2010 August 7. Analyzing this evolution, we obtain the following main
results. 1) During hours before the eruption, filament threads within the lower
branch were observed to intermittently brighten up, lift upward, and then merge
with the upper branch. The merging process contributed magnetic flux and
current to the upper branch, resulting in its quasi-static ascent. 2) This
transfer might serve as the key mechanism for the upper branch to lose
equilibrium by reaching the limiting flux that can be stably held down by the
overlying field or by reaching the threshold of the torus instability. 3) The
erupting branch first straightened from a reverse S shape that followed the
polarity inversion line and then writhed into a forward S shape. This shows a
transfer of left-handed helicity in a sequence of writhe-twist-writhe. The fact
that the initial writhe is converted into the twist of the flux rope excludes
the helical kink instability as the trigger process of the eruption, but
supports the occurrence of the instability in the main phase, which is indeed
indicated by the very strong writhing motion. 4) A hard X-ray sigmoid, likely
of coronal origin, formed in the gap between the two original filament branches
in the impulsive phase of the associated flare. This supports a model of
transient sigmoids forming in the vertical flare current sheet. 5) Left-handed
magnetic helicity is inferred for both branches of the dextral filament. 6) Two
types of force-free magnetic configurations are compatible with the data, a
double flux rope equilibrium and a single flux rope situated above a loop
arcade.
