According to research into the complex and rich history of Chinese music, there is data that indicates that the term "qin" was used generically for the zither family (currently guqin, "gu" in Mandarin means "old"). The directors of "The Double" were extremely careful when using the term "qin", as this type of instrument was commonly called in that historical period (Song Dynasty), as it belongs to the family of plucked instruments, even though it is a "zheng", with 13 strings, it is possible to count the 13 strings, I took a print of the scene and counted the strings  (guzheng is the modern version with 21 strings).  

Qin" (guqin...) is a very subtle instrument, which leads the practitioner to a deep introspection, requires a performance on a contemplative level, Confucius, for example, played the "qin". You all may have already noticed that the "qin" is the musical instrument that appears most in Chinese series and films and is always performed by literati. In the case of The Double, XFF isn't a literati, but a woman with a good family and educational background. Learning a musical instrument was an important part of children's education, giving them prominence within society.  Furthermore, the sound/timbral characteristics of "zheng" are completely different from those of "qin", making it impossible to get confused. 

The directors had no intention of using the "zheng" for a better plastic effect or as a cinematography element, but merely using an instrument that has a much richer and more complex musical (harmonic) capacity than a "qin" which has only 7 strings and it was most used by literati, it was an instrument of intellectual and spiritual cultivation. The "zheng" was more appropriate for a competition, as it offered greater technical, melodic and harmonic possibilities. If anyone is interested in the musical piece that XFF plays in "zheng", I did a thorough research to find out its real title, its composer and other information. The link is below this comment.
All these details demonstrate the seriousness of the series' direction in dealing with China's ancient history). I'm particularly interested in musical aspects because I'm a musician (pianist, flutist, lyrical singer) and a professor of music and art history, I have a deep admiration for Chinese civilization, I dedicate myself to its research. It's rare to see such seriousness in the film industry in reproducing a historical fact of music in China. These details and countless others are what make "The Double" a sophisticated work, a masterpiece that has in its creation elements of Chinese opera, ancient scroll painting, among other artistic languages, in addition to the subtle use of semiotics (symbology) in its non-explicit narrative that forces us to exercise our ability to unravel real charades.

https://mydramalist.com/discussions/di-jia-qian-jin/127205-all-information-about-the-song-jiang-li-played-on-the-guzheng-piano-sheet-music

I appreciate when art work is done with devotion. There are so many details to explore. Thank you for the clarification of the 'qin' instrument in historical perspective!

I'm very grateful for your comment here and in the MDL comments, you're a sensitive and polite person. It's a pleasure to exchange points of view with a person who seeks qualified information and has a love for culture. In fact, culture and the Beauty that emanates from it, is a source of joy and a door that opens universes to
to be explored! Thank you very much for the conversation I had with you that gave rise to this publication with information about "qin", "zheng" and the seriousness of the directors of The Double in dealing with historical facts that educate everyone that this series captivates!