DuckDuckGo does not have its own index. They are pretty much layer over Bing plus some customizations for query parsing and head queries for trusted sources which they do index. If Bing decided to block DDG tomorrow, they will be toast. The only impressive thing for me about DDG is that they are able to turn privacy thing in to marketing for geeks and get tiny trickle of traffic to stay alive in hope someone would buy them out.