Not on the Duo specifically, but some thoughts none-the-less.

Best to use a pressure pot (can be a good quality pump oil can, but
better is a small pump-up garden sprayer modified for the job, as this
allows some serious through-put to overcome and move out bubbles in
high spots).
Cycle the brake lever a few times while pumping the fluid up.

Note that some installations don't have the brake line going into the
caliper at the top, thus there can be a stubborn bubble in the caliper
itself. In this case, remove caliper and but a clamp on piston to keep
it from coming out. Then bleed with brake line at top.

To get more through put, you will need remove all brake fluid from the
reservoir with a syringe. Better is to intall a fitting in the
existing cap vent hole so you can install a temporary tube to take off
the excess brake fluid. Depending on existing cap vent hole, you may
be able to use a large hypodermic needle for this (McMaster-Car sells
them 1/8" OD) or you can drill hole bigger and install a nipple to
accept the vent tube. Pump brake fluid up until no more bubbles exit
vent tube - - which can be placed in a container so bubbles are
clearly visible.

Interesting that the Duo uses automotive Dot fluid?? I know the Stemme
does also. Schleicher uses the more common (at least in the USA for
small GA) Mil Spec 5606 fluid (the good ole red stuff). You cannot mix
or substitute the two as 5606 calls for Buna-N seals and O-rings,
while Dot fluid takes EPDM rubber seals.

Tost buys some US Cleavland brake parts, such as the 30- series
caliper, and replaces the Buna-N O-rings with EPDM. The Cleveland
calipers, and their early non-anodized pistons, tend to corrode due to
moisture as the Dot fluids are hygroscopic. Later pistons, after the
mid-90's or so, are anodized, but the caliper bore is still prone to
corrode when Dot fluids are used, and this leads to leaks and poor
braking. So be sure to check caliper bore condition if using Dot
fluids. The long term fix is to sleave the bores with brass liners and
anodize the pistons if they're the early ones.

bumper
MKIV & QV
Minden