Allow extending more than once sections extending other sections.

src/zc/buildout/buildout.py

@@ -1153,6 +1153,7 @@ class Options(UserDict.DictMixin):
 
         if '<' in self._raw:
             self._raw = self._do_extend_raw(name, self._raw, [])
+            self._raw.pop('<')
 
         # force substitutions
         for k, v in self._raw.items():
@@ -1181,7 +1182,7 @@ class Options(UserDict.DictMixin):
             raise zc.buildout.UserError("Infinite extending loop %r" % name)
         doing.append(name)
         try:
-            to_do = data.pop('<', None)
+            to_do = data.get('<', None)
             if to_do is None:
                 return data
             __doing__ = 'Loading input sections for %r', name

src/zc/buildout/tests.py

@@ -3587,6 +3587,52 @@ def increment_on_command_line():
       recipe='zc.buildout:debug'
     """
 
+def bug_605017_reuse_extending_section():
+    r"""
+    >>> write('buildout.cfg', '''
+    ... [buildout]
+    ... parts = myfiles
+    ...   myfiles2
+    ...
+    ... [debug]
+    ... recipe = zc.buildout:debug
+    ...
+    ... [with_file1]
+    ... <= debug
+    ... file1 = ${:path}/file1
+    ... color = red
+    ...
+    ... [with_file2]
+    ... <= debug
+    ... file2 = ${:path}/file2
+    ... color = blue
+    ...
+    ... [myfiles]
+    ... <= with_file1
+    ...    with_file2
+    ... path = mydata
+    ...
+    ... [myfiles2]
+    ... <= with_file1
+    ...    with_file2
+    ... path = mydata2
+    ... ''')
+
+    >>> print system(buildout),
+    Installing myfiles.
+      color='blue'
+      file1='mydata/file1'
+      file2='mydata/file2'
+      path='mydata'
+      recipe='zc.buildout:debug'
+    Installing myfiles2.
+      color='blue'
+      file1='mydata2/file1'
+      file2='mydata2/file2'
+      path='mydata2'
+      recipe='zc.buildout:debug'
+    """
+
 ######################################################################
 
 def make_py_with_system_install(make_py, sample_eggs):