Fixed a parse bug with the html5lib tree-builder. Thanks to Roel

  Kramer for the patch. [bug=1483781]

3 files changed, 14 insertions, 1 deletions

diff --git a/NEWS.txt b/NEWS.txt
index 59f3b44..3726c57 100644
--- a/NEWS.txt
+++ b/NEWS.txt
@@ -4,6 +4,9 @@
   removed. Thanks to Eric Weiser for the patch and John Wiseman for a
   test. [bug=1481520]
 
+* Fixed a parse bug with the html5lib tree-builder. Thanks to Roel
+  Kramer for the patch. [bug=1483781]
+
 * Improved the implementation of CSS selector grouping. Thanks to
   Orangain for the patch. [bug=1484543]
 
diff --git a/bs4/builder/_html5lib.py b/bs4/builder/_html5lib.py
index ab5793c..8725a65 100644
--- a/bs4/builder/_html5lib.py
+++ b/bs4/builder/_html5lib.py
@@ -120,7 +120,10 @@ class AttrList(object):
         if (name in list_attr['*']
             or (self.element.name in list_attr
                 and name in list_attr[self.element.name])):
-            value = whitespace_re.split(value)
+            # A node that is being cloned may have already undergone
+            # this procedure.
+            if not isinstance(value, list):
+                value = whitespace_re.split(value)
         self.element[name] = value
     def items(self):
         return list(self.attrs.items())
diff --git a/bs4/tests/test_html5lib.py b/bs4/tests/test_html5lib.py
index 9a2bacf..65536c2 100644
--- a/bs4/tests/test_html5lib.py
+++ b/bs4/tests/test_html5lib.py
@@ -89,3 +89,10 @@ class HTML5LibBuilderSmokeTest(SoupTest, HTML5TreeBuilderSmokeTest):
         markup = b"""<?PITarget PIContent?>"""
         soup = self.soup(markup)
         assert str(soup).startswith("<!--?PITarget PIContent?-->")
+
+    def test_cloned_multivalue_node(self):
+        markup = b"""<a class="my_class"><p></a>"""
+        soup = self.soup(markup)
+        a1, a2 = soup.find_all('a')
+        self.assertEqual(a1, a2)
+        assert a1 is not a2