The Duqu Trojan was first detected in September 2011, but Kaspersky Lab believes it has seen the first pieces of Duqu-related malware dating back to August 2007. The code was most likely written with a custom extension to C, generally called “OO C” The C&C code could have been reused from an already existing software project and integrated into the Duqu malware. It’s believed the developers are old school that don’t trust C++ and that’s probably why they relied on C.
Source: https://thehackernews.com/2012/03/mystery-of-duqu-programming-language.html