  Partial eruptions of solar filaments are the typical representative of solar
eruptive behavior diversity. Here we investigate a typical filament partial
eruption event and present integrated evidence for configuration of the
pre-eruption filament and its formation. The CHASE H$\alpha$ observations
reveal structured Doppler velocity distribution within the pre-eruption
filament, where distinct redshift only appeared in the east narrow part of the
south filament region and then disappeared after the partial eruption while the
north part dominated by blueshift remained. Combining the SDO, ASO-S
observations, and NLFFF modeling results, we verify that there were two
independent material flow systems within the pre-flare filament, whose magnetic
topology is a special double-decker configuration consisting of two magnetic
flux ropes (MFRs) with opposite magnetic twist. During the formation of this
filament system, continuous magnetic flux cancellation and footpoint motion
were observed around its north end. Therefore, we propose a new double-decker
formation scenario that the two MFRs composing such double-decker configuration
originated from two magnetic systems with different initial connections and
opposite magnetic twist. Subsequent magnetic reconnection with surrounding
newly-emerging fields resulted in the motion of footpoint of the upper MFR to
the region around footpoint of the lower MFR, thus leading to eventual
formation of the double-decker configuration consisting of two MFRs with
similar footpoints but opposite signs of magnetic twist. These results provide
a potential way to determine unambiguously the progenitor configuration of a
partial-eruptive filament and reveal a special type of double-decker MFR
configuration and a new double-decker formation scenario.
