Query string (request) parameters can have multiple values. See [ServletRequest#getParameterValues(String)](http://docs.oracle.com/javaee/6/api/javax/servlet/ServletRequest.html#getParameterValues%28java.lang.String%29).

Too bad there's no tool for serializing a ``Map<String, [String]>`` or ``Map<String, List<String>>`` to a properly encoded query string. I had to do this manually too in [Solr Search UI](https://github.com/xwiki/xwiki-platform/blob/xwiki-platform-5.1/xwiki-platform-core/xwiki-platform-search/xwiki-platform-search-solr/xwiki-platform-search-solr-ui/src/main/resources/Main/SolrSearch.xml#L1105).