diff --git a/pip/req/req_set.py b/pip/req/req_set.py
index 2feb25e4c71..74a86752979 100644
--- a/pip/req/req_set.py
+++ b/pip/req/req_set.py
@@ -241,7 +241,8 @@ def add_requirement(self, install_req, parent_req_name=None):
             except KeyError:
                 existing_req = None
             if (parent_req_name is None and existing_req and not
-                    existing_req.constraint):
+                    existing_req.constraint and
+                    existing_req.extras == install_req.extras):
                 raise InstallationError(
                     'Double requirement given: %s (already in %s, name=%r)'
                     % (install_req, existing_req, name))
@@ -267,6 +268,11 @@ def add_requirement(self, install_req, parent_req_name=None):
                     # If we're now installing a constraint, mark the existing
                     # object for real installation.
                     existing_req.constraint = False
+                    existing_req.extras = tuple(
+                        sorted(set(existing_req.extras).union(
+                               set(install_req.extras))))
+                    logger.debug("Setting %s extras to: %s" % (
+                        existing_req, existing_req.extras))
                     # And now we need to scan this.
                     result = [existing_req]
                 # Canonicalise to the already-added object for the backref
diff --git a/tests/data/packages/LocalExtras-0.0.2/.gitignore b/tests/data/packages/LocalExtras-0.0.2/.gitignore
new file mode 100644
index 00000000000..bc186a3f1cf
--- /dev/null
+++ b/tests/data/packages/LocalExtras-0.0.2/.gitignore
@@ -0,0 +1 @@
+/LocalExtras-0.0.2.egg-info
diff --git a/tests/data/packages/LocalExtras-0.0.2/localextras/__init__.py b/tests/data/packages/LocalExtras-0.0.2/localextras/__init__.py
new file mode 100644
index 00000000000..e69de29bb2d
diff --git a/tests/data/packages/LocalExtras-0.0.2/setup.py b/tests/data/packages/LocalExtras-0.0.2/setup.py
new file mode 100644
index 00000000000..a32e344f5f0
--- /dev/null
+++ b/tests/data/packages/LocalExtras-0.0.2/setup.py
@@ -0,0 +1,30 @@
+import os
+from setuptools import setup, find_packages
+
+
+def path_to_url(path):
+    """
+    Convert a path to URI. The path will be made absolute and
+    will not have quoted path parts.
+    """
+    path = os.path.normpath(os.path.abspath(path))
+    drive, path = os.path.splitdrive(path)
+    filepath = path.split(os.path.sep)
+    url = '/'.join(filepath)
+    if drive:
+        return 'file:///' + drive + url
+    return 'file://' +url
+
+
+HERE = os.path.dirname(__file__)
+DEP_PATH = os.path.join(HERE, '..', '..', 'indexes', 'simple', 'simple')
+DEP_URL = path_to_url(DEP_PATH)
+
+setup(
+    name='LocalExtras',
+    version='0.0.2',
+    packages=find_packages(),
+    install_requires=['simple==1.0'],
+    extras_require={ 'bar': ['simple==2.0'], 'baz': ['singlemodule'] },
+    dependency_links=[DEP_URL]
+)
diff --git a/tests/data/packages/LocalExtras/setup.py b/tests/data/packages/LocalExtras/setup.py
index 7c2839819a8..fea6e16039a 100644
--- a/tests/data/packages/LocalExtras/setup.py
+++ b/tests/data/packages/LocalExtras/setup.py
@@ -24,6 +24,6 @@ def path_to_url(path):
     name='LocalExtras',
     version='0.0.1',
     packages=find_packages(),
-    extras_require={ 'bar': ['simple'] },
+    extras_require={ 'bar': ['simple'], 'baz': ['singlemodule'] },
     dependency_links=[DEP_URL]
 )
diff --git a/tests/functional/test_install_reqs.py b/tests/functional/test_install_reqs.py
index 5c2ed0c6493..f30d0852655 100644
--- a/tests/functional/test_install_reqs.py
+++ b/tests/functional/test_install_reqs.py
@@ -343,3 +343,95 @@ def test_double_install_spurious_hash_mismatch(script, tmpdir):
         result = script.pip_install_local(
             '-r', reqs_file.abspath, expect_error=False)
         assert 'Successfully installed simple-1.0' in str(result)
+
+
+def test_install_with_extras_from_constraints(script, data):
+    to_install = data.packages.join("LocalExtras")
+    script.scratch_path.join("constraints.txt").write(
+        "file://%s#egg=LocalExtras[bar]" % to_install
+    )
+    result = script.pip_install_local(
+        '-c', script.scratch_path / 'constraints.txt', 'LocalExtras')
+    assert script.site_packages / 'simple' in result.files_created
+
+
+def test_install_with_extras_from_install(script, data):
+    to_install = data.packages.join("LocalExtras")
+    script.scratch_path.join("constraints.txt").write(
+        "file://%s#egg=LocalExtras" % to_install
+    )
+    result = script.pip_install_local(
+        '-c', script.scratch_path / 'constraints.txt', 'LocalExtras[baz]')
+    assert script.site_packages / 'singlemodule.py'in result.files_created
+
+
+def test_install_with_extras_joined(script, data):
+    to_install = data.packages.join("LocalExtras")
+    script.scratch_path.join("constraints.txt").write(
+        "file://%s#egg=LocalExtras[bar]" % to_install
+    )
+    result = script.pip_install_local(
+        '-c', script.scratch_path / 'constraints.txt', 'LocalExtras[baz]'
+    )
+    assert script.site_packages / 'simple' in result.files_created
+    assert script.site_packages / 'singlemodule.py'in result.files_created
+
+
+def test_install_with_extras_editable_joined(script, data):
+    to_install = data.packages.join("LocalExtras")
+    script.scratch_path.join("constraints.txt").write(
+        "-e file://%s#egg=LocalExtras[bar]" % to_install
+    )
+    result = script.pip_install_local(
+        '-c', script.scratch_path / 'constraints.txt', 'LocalExtras[baz]')
+    assert script.site_packages / 'simple' in result.files_created
+    assert script.site_packages / 'singlemodule.py'in result.files_created
+
+
+def test_install_distribution_full_union(script, data):
+    to_install = data.packages.join("LocalExtras")
+    result = script.pip_install_local(
+        to_install, to_install + "[bar]", to_install + "[baz]")
+    assert 'Running setup.py install for LocalExtras' in result.stdout
+    assert script.site_packages / 'simple' in result.files_created
+    assert script.site_packages / 'singlemodule.py' in result.files_created
+
+
+def test_install_distribution_duplicate_extras(script, data):
+    to_install = data.packages.join("LocalExtras")
+    package_name = to_install + "[bar]"
+    with pytest.raises(AssertionError):
+        result = script.pip_install_local(package_name, package_name)
+        assert 'Double requirement given: %s' % package_name in result.stderr
+
+
+def test_install_distribution_union_with_constraints(script, data):
+    to_install = data.packages.join("LocalExtras")
+    script.scratch_path.join("constraints.txt").write(
+        "%s[bar]" % to_install)
+    result = script.pip_install_local(
+        '-c', script.scratch_path / 'constraints.txt', to_install + '[baz]')
+    assert 'Running setup.py install for LocalExtras' in result.stdout
+    assert script.site_packages / 'singlemodule.py' in result.files_created
+
+
+def test_install_distribution_union_with_versions(script, data):
+    to_install_001 = data.packages.join("LocalExtras")
+    to_install_002 = data.packages.join("LocalExtras-0.0.2")
+    result = script.pip_install_local(
+        to_install_001 + "[bar]", to_install_002 + "[baz]")
+    assert ("Successfully installed LocalExtras-0.0.1 simple-3.0 " +
+            "singlemodule-0.0.1" in result.stdout)
+
+
+@pytest.mark.xfail
+def test_install_distribution_union_conflicting_extras(script, data):
+    # LocalExtras requires simple==1.0, LocalExtras[bar] requires simple==2.0;
+    # without a resolver, pip does not detect the conflict between simple==1.0
+    # and simple==2.0. Once a resolver is added, this conflict should be
+    # detected.
+    to_install = data.packages.join("LocalExtras-0.0.2")
+    result = script.pip_install_local(to_install, to_install + "[bar]",
+                                      expect_error=True)
+    assert 'installed' not in result.stdout
+    assert "Conflict" in result.stderr