[fix] Check that sections don't install the same path

src/zc/buildout/buildout.py

@@ -937,7 +937,9 @@ class Buildout(DictMixin):
         # Check for unused buildout options:
         _check_for_unused_options_in_section(self, 'buildout')
 
+
         # install new parts
+        all_installed_paths = {}
         for part in install_parts:
             signature = self[part].pop('__buildout_signature__')
             saved_options = self[part].copy()
@@ -968,11 +970,11 @@ class Buildout(DictMixin):
                         ' '.join(installed_parts))
                     raise
 
+                installed_files = set(installed_files.split('\n')) \
+                    if installed_files else set()
                 if updated_files:
-                    installed_files = set(installed_files.split('\n'))
                     (installed_files.add if isinstance(updated_files, str) else
                      installed_files.update)(updated_files)
-                    installed_files = '\n'.join(sorted(installed_files))
 
             else: # install
                 __doing__ = 'Installing %s.', part
@@ -985,9 +987,23 @@ class Buildout(DictMixin):
                         "The %s install returned None.  A path or "
                         "iterable os paths should be returned.",
                         part)
-                    installed_files = ""
-                elif not isinstance(installed_files, str):
-                    installed_files = '\n'.join(installed_files)
+                elif installed_files:
+                    installed_files = ({installed_files}
+                        if isinstance(installed_files, str) else
+                        set(installed_files))
+
+            if installed_files:
+                conflicts = installed_files.intersection(
+                    all_installed_paths)
+                if conflicts:
+                    self._error(
+                        "The following paths are already"
+                        " installed by other sections: %r",
+                        {x: all_installed_paths[x] for x in conflicts})
+                all_installed_paths.update(dict.fromkeys(installed_files, part))
+                installed_files = '\n'.join(sorted(installed_files))
+            else:
+                installed_files = ''
 
             saved_options['__buildout_installed__'] = installed_files
             saved_options['__buildout_signature__'] = signature