diff --git a/scrapy/http/request/form.py b/scrapy/http/request/form.py
index af4ed1793..b6ca6ef9c 100644
--- a/scrapy/http/request/form.py
+++ b/scrapy/http/request/form.py
@@ -34,11 +34,8 @@ class FormRequest(Request):
                 self._set_body(querystr)
             else:
                 url_split = urlsplit(self.url)
-                formdata_key_list = []
-                for k in querystr.split('&'):
-                    formdata_key_list.append(k.split('=')[0])
-                items = []
-                items += [(k, v) for k, v in parse_qsl(url_split.query) if k not in formdata_key_list]
+                formdata_key_list = list(dict(parse_qsl(querystr)).keys())
+                items = [(k, v) for k, v in parse_qsl(url_split.query) if k not in formdata_key_list]
                 query_str = _urlencode(items, self.encoding)
                 self._set_url(urljoin(self.url,'?'+ (query_str + '&' if query_str else '') + querystr + ('#'+ url_split.fragment if url_split.fragment else '')))