diff --git a/news/6239.bugfix b/news/6239.bugfix
new file mode 100644
index 00000000000..d157641e838
--- /dev/null
+++ b/news/6239.bugfix
@@ -0,0 +1,2 @@
+Always try to merge extras from same-named requirements to prevent ``package``
+from shadowing ``package[extra]``.
diff --git a/src/pip/_internal/req/req_set.py b/src/pip/_internal/req/req_set.py
index d1410e93514..e1dea071173 100644
--- a/src/pip/_internal/req/req_set.py
+++ b/src/pip/_internal/req/req_set.py
@@ -130,11 +130,27 @@ def add_requirement(
             # We'd want to rescan this requirements later
             return [install_req], install_req
 
-        # Assume there's no need to scan, and that we've already
-        # encountered this for scanning.
-        if install_req.constraint or not existing_req.constraint:
+        # If the incoming requirement is a constraint, its job is done.
+        if install_req.constraint:
             return [], existing_req
 
+        # Merge extras from existing and incoming requirements. This needs to
+        # be done always to avoid an existing req "swallowing" new extras given
+        # by a new req. (pypa/pip#6239)
+        cur_extras = set(existing_req.extras)
+        new_extras = set(install_req.extras) | cur_extras
+        if new_extras != cur_extras:
+            existing_req.extras = tuple(sorted(new_extras))
+            logger.debug(
+                "Setting %s extras to: %s",
+                existing_req, existing_req.extras,
+            )
+
+        # If the existing requirement is NOT a constraint, we're done.
+        # Otherwise the incoming one needs to be checked against it.
+        if not existing_req.constraint:
+            return [existing_req], existing_req
+
         does_not_satisfy_constraint = (
             install_req.link and
             not (
@@ -152,13 +168,7 @@ def add_requirement(
         # 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) | set(install_req.extras)
-        ))
-        logger.debug(
-            "Setting %s extras to: %s",
-            existing_req, existing_req.extras,
-        )
+
         # Return the existing requirement for addition to the parent and
         # scanning again.
         return [existing_req], existing_req
diff --git a/tests/unit/test_req.py b/tests/unit/test_req.py
index 32cc1b36d2d..515e07fba9d 100644
--- a/tests/unit/test_req.py
+++ b/tests/unit/test_req.py
@@ -316,6 +316,33 @@ def test_hashed_deps_on_require_hashes(self):
             lineno=2
         ))
 
+    @pytest.mark.parametrize(
+        ('req1_constraint', 'req2_constraint', 'expected_extras'),
+        [
+            (False, False, ['jwt']),
+            (True, False, ['jwt']),
+            (False, True, []),
+            (True, True, []),
+        ],
+    )
+    def test_add_requirement_extend_extra(
+            self, req1_constraint, req2_constraint, expected_extras):
+        """Make sure adding a requirement with extra merges it with
+        an existing one with the same name (if exists).
+
+        See pypa/pip#6239.
+        """
+        req1 = get_processed_req_from_line('boxsdk', lineno=1)
+        req1.constraint = req1_constraint
+
+        req2 = get_processed_req_from_line('boxsdk[jwt]', lineno=1)
+        req2.constraint = req2_constraint
+
+        reqset = RequirementSet()
+        reqset.add_requirement(req1)
+        reqset.add_requirement(req2)
+        assert list(reqset.get_requirement('boxsdk').extras) == expected_extras
+
 
 @pytest.mark.parametrize(('file_contents', 'expected'), [
     (b'\xf6\x80', b'\xc3\xb6\xe2\x82\xac'),  # cp1252
diff --git a/tests/yaml/install/extras.yml b/tests/yaml/install/extras.yml
index ee23b5e4af0..e813a2becce 100644
--- a/tests/yaml/install/extras.yml
+++ b/tests/yaml/install/extras.yml
@@ -39,4 +39,3 @@ cases:
       - D 1.0.0
       - E 1.0.0
       - F 1.0.0
-  skip: true