How can I filter unique lines based on a substring?

If I have a set of URLs in a file like:

http://example.com/do?foo=bar&etc=123

http://example.com/do?etc=456&foo=bar

I want to filter this out so that there's only one url for each unique value of the 'foo' parameter. The rest of the URL doesn't matter, the only important thing to me is that I don't have two or more lines with the same value of 'foo'.