diff --git a/prometheus_client/core.py b/prometheus_client/core.py
index 840fc9b2..340d955a 100644
--- a/prometheus_client/core.py
+++ b/prometheus_client/core.py
@@ -48,10 +48,11 @@ def register(self, collector):
         '''Add a collector to the registry.'''
         with self._lock:
             names = self._get_names(collector)
-            for name in names:
-                if name in self._names_to_collectors:
-                    raise ValueError('Timeseries already present '
-                            'in CollectorRegistry: ' + name)
+            duplicates = set(self._names_to_collectors).intersection(names)
+            if duplicates:
+                raise ValueError(
+                    'Duplicated timeseries in CollectorRegistry: {}'.format(
+                        duplicates))
             for name in names:
                 self._names_to_collectors[name] = collector
             self._collector_to_names[collector] = names
@@ -444,7 +445,8 @@ def __reset(self):
                 file_prefix = typ
             if file_prefix not in files:
                 filename = os.path.join(
-                        os.environ['prometheus_multiproc_dir'], '{0}_{1}.db'.format(file_prefix, pid['value']))
+                    os.environ['prometheus_multiproc_dir'],
+                    '{0}_{1}.db'.format(file_prefix, pid['value']))
                 files[file_prefix] = _MmapedDict(filename)
             self._file = files[file_prefix]
             self._key = json.dumps((metric_name, name, labelnames, labelvalues))
@@ -710,8 +712,9 @@ def __init__(self, name, labelnames, labelvalues, multiprocess_mode='all'):
         if (_ValueClass._multiprocess
                 and multiprocess_mode not in ['min', 'max', 'livesum', 'liveall', 'all']):
             raise ValueError('Invalid multiprocess mode: ' + multiprocess_mode)
-        self._value = _ValueClass(self._type, name, name, labelnames,
-                labelvalues, multiprocess_mode=multiprocess_mode)
+        self._value = _ValueClass(
+            self._type, name, name, labelnames, labelvalues,
+            multiprocess_mode=multiprocess_mode)
 
     def inc(self, amount=1):
         '''Increment gauge by the given amount.'''
diff --git a/prometheus_client/exposition.py b/prometheus_client/exposition.py
index a2ba8fbc..aff3bffb 100644
--- a/prometheus_client/exposition.py
+++ b/prometheus_client/exposition.py
@@ -286,8 +286,9 @@ def _use_gateway(method, gateway, job, registry, grouping_key, timeout, handler)
 
     if grouping_key is None:
         grouping_key = {}
-    url = url + ''.join(['/{0}/{1}'.format(quote_plus(str(k)), quote_plus(str(v)))
-                             for k, v in sorted(grouping_key.items())])
+    url += ''.join(
+        '/{0}/{1}'.format(quote_plus(str(k)), quote_plus(str(v)))
+        for k, v in sorted(grouping_key.items()))
 
     headers=[('Content-Type', CONTENT_TYPE_LATEST)]
     handler(url=url, method=method, timeout=timeout,
diff --git a/tests/test_multiprocess.py b/tests/test_multiprocess.py
index 8f282d18..916b4457 100644
--- a/tests/test_multiprocess.py
+++ b/tests/test_multiprocess.py
@@ -153,8 +153,9 @@ def test_multi_expansion(self):
         self.d.write_value('abc', 42.0)
         self.d.write_value(key, 123.0)
         self.d.write_value('def', 17.0)
-        self.assertEqual([('abc', 42.0), (key, 123.0), ('def', 17.0)],
-                list(self.d.read_all_values()))
+        self.assertEqual(
+            [('abc', 42.0), (key, 123.0), ('def', 17.0)],
+            list(self.d.read_all_values()))
 
     def tearDown(self):
         os.unlink(self.tempfile)
diff --git a/tox.ini b/tox.ini
index 1e3b6e24..828539ff 100644
--- a/tox.ini
+++ b/tox.ini
@@ -63,9 +63,6 @@ ignore =
     E111,
     E114,
     E123,
-    E126,
-    E127,
-    E128,
     E231,
     E222,
     E225,