oh wait its not ducked.. but I used to do something close to this effect. Not totally it, but close.
It was a bit cumbersome but it was like this:
DSP/FX Bypass mode.
EXP set to MIX 0-100
Usually an very long type delay
play the line I want to repeat
BYPASS out. the line continues. At this point andy thing I play is Dry, and the EXP controls the volume of the previous repeat.
If I want some more live prepeast, I just activate in, start at dry-mix and fade in.
YOu could toggle the BYPASS switch to throw certain notes in, as well.
umbersome, It is true