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Are there any apps that perform searches on multiple search engines and combine the results?

You are looking for SearX. Its a meta search engine and really hackable. Also, its open source.

You mean a meta-engine? Like Duckduckgo? Or you actually mean an app you can install?

Does ddg use google results? I have read that they do not.

They use Google's index I believe, but that is not the same as using Google's results.