Check that each version appears once only

Yet another check for merge issues - the combination of empty lines and
dash over- and underlines for titles seems to confuse git regularly.

scripts/check-changelog.py

@@ -61,3 +61,11 @@
     if all_versions != sorted(all_versions):
         print('You edited old release notes and changed the ordering!')
         sys.exit(1)
+
+    v_nums = [v.split(' - ')[0] for v in all_versions]
+    duplicates = sorted(set(v for v in v_nums if v_nums.count(v) > 1))
+    if duplicates:
+        plural = 's have' if len(duplicates) > 1 else ' has'
+        print('The following version%s multiple entries in the '
+              'changelog: %s' % (plural, ', '.join(map(repr, duplicates))))
+        sys.exit(1)