I'm not aware of any transmissions on the Renault Captur that are called "duplex"... I think the next big automatic for that range will be a CVT.
Usually, "duplex" refers to a manual transmission mostly used in trucks, which consists of two distinct coutnershafts which are selectable. Mack B series and most notable the Eaton-Fuller "Roadranger" series with which I have some rowing time. These transmissions have a selection of main gears, augmented with another whole set of gears; a type of HI-LO arrangement.
This becomes quite intense with triplex and quadraplex gearsets... (the second part of the transmission having LOLO, LO, DIRECT, and HI positions, with as many 5 "main" transmission ratios.) The shift pattern on say a Mack 15, 18, or 20 speed is a work of long practice and mastery, which involves a complex ballet to acheive every possible ratio in perfect ascention or decension. In practice, one could skip a few (especially unloaded) as many ratios were only a small fraction different than their nearest next lower or higher counterparts.
"Semiautomatic" usually refers to a system that allows gear selection without, or with limited use of, a clutch. I once owned a 1950 Dodge Wayfarer with such an arrangement - at the time called the first semi-automatic transmission.
The transmission was a combination of manual and "automatic" elements, much like a modern automatic transmission. However, gear changes were performed manually, not as a result of fluid pressure or RPM.