Or people could append random query parameters, HN would have no way of detecting that http://example.com/?foo http://example.com/?foo, http://example.com/?bar

Or people could append random query parameters, HN would have no way of detecting that http://example.com/?foo http://example.com/?foo, http://example.com/?bar http://example.com/?bar and http://example.com/?baz http://example.com/?baz points to the same article. No need to change anything on the webpage.