I thought that DDG used Yahoo! Search under the hood exclusively (because... why use more than one?). It turns out that it is a complex mix of resources from multiple partners[0]:
> In July 2016, DuckDuckGo officially announced the extension of its partnership with Yahoo! that brought new features to all users of the search engine, including date filtering of results and additional site links. It also partners with Bing, Yandex, and Wikipedia to produce results or make use of features offered. The company also confirmed that it does not share user information with partner companies, as has always been its policy.