diff --git a/src/pip/_internal/req/__init__.py b/src/pip/_internal/req/__init__.py
index 75b532b0e..8568d3f8b 100644
--- a/src/pip/_internal/req/__init__.py
+++ b/src/pip/_internal/req/__init__.py
@@ -1,8 +1,6 @@
-# The following comment should be removed at some point in the future.
-# mypy: strict-optional=False
-
 from __future__ import absolute_import
 
+import collections
 import logging
 
 from pip._internal.utils.logging import indent_log
@@ -13,7 +11,7 @@ from .req_install import InstallRequirement
 from .req_set import RequirementSet
 
 if MYPY_CHECK_RUNNING:
-    from typing import List, Optional, Sequence
+    from typing import Iterator, List, Optional, Sequence, Tuple
 
 __all__ = [
     "RequirementSet", "InstallRequirement",
@@ -33,8 +31,17 @@ class InstallationResult(object):
         return "InstallationResult(name={!r})".format(self.name)
 
 
+def _validate_requirements(
+    requirements,  # type: List[InstallRequirement]
+):
+    # type: (...) -> Iterator[Tuple[str, InstallRequirement]]
+    for req in requirements:
+        assert req.name, "invalid to-be-installed requirement: {}".format(req)
+        yield req.name, req
+
+
 def install_given_reqs(
-    to_install,  # type: List[InstallRequirement]
+    requirements,  # type: List[InstallRequirement]
     install_options,  # type: List[str]
     global_options,  # type: Sequence[str]
     root,  # type: Optional[str]
@@ -50,23 +57,27 @@ def install_given_reqs(
 
     (to be called after having downloaded and unpacked the packages)
     """
+    to_install = collections.OrderedDict(_validate_requirements(requirements))
 
     if to_install:
         logger.info(
             'Installing collected packages: %s',
-            ', '.join([req.name for req in to_install]),
+            ', '.join(to_install.keys()),
         )
 
     installed = []
 
     with indent_log():
-        for requirement in to_install:
+        for req_name, requirement in to_install.items():
             if requirement.should_reinstall:
-                logger.info('Attempting uninstall: %s', requirement.name)
+                logger.info('Attempting uninstall: %s', req_name)
                 with indent_log():
                     uninstalled_pathset = requirement.uninstall(
                         auto_confirm=True
                     )
+            else:
+                uninstalled_pathset = None
+
             try:
                 requirement.install(
                     install_options,
@@ -79,22 +90,14 @@ def install_given_reqs(
                     pycompile=pycompile,
                 )
             except Exception:
-                should_rollback = (
-                    requirement.should_reinstall and
-                    not requirement.install_succeeded
-                )
                 # if install did not succeed, rollback previous uninstall
-                if should_rollback:
+                if uninstalled_pathset and not requirement.install_succeeded:
                     uninstalled_pathset.rollback()
                 raise
             else:
-                should_commit = (
-                    requirement.should_reinstall and
-                    requirement.install_succeeded
-                )
-                if should_commit:
+                if uninstalled_pathset and requirement.install_succeeded:
                     uninstalled_pathset.commit()
 
-            installed.append(InstallationResult(requirement.name))
+            installed.append(InstallationResult(req_name))
 
     return installed