diff --git a/src/pip/_internal/resolution/resolvelib/candidates.py b/src/pip/_internal/resolution/resolvelib/candidates.py
index 1a048eb10..f8461ade2 100644
--- a/src/pip/_internal/resolution/resolvelib/candidates.py
+++ b/src/pip/_internal/resolution/resolvelib/candidates.py
@@ -288,6 +288,17 @@ class AlreadyInstalledCandidate(Candidate):
             distribution=self.dist,
         )
 
+    def __eq__(self, other):
+        # type: (Any) -> bool
+        if isinstance(other, self.__class__):
+            return self.name == other.name and self.version == other.version
+        return False
+
+    # Needed for Python 2, which does not implement this by default
+    def __ne__(self, other):
+        # type: (Any) -> bool
+        return not self.__eq__(other)
+
     @property
     def name(self):
         # type: () -> str
@@ -351,6 +362,17 @@ class ExtrasCandidate(Candidate):
             extras=self.extras,
         )
 
+    def __eq__(self, other):
+        # type: (Any) -> bool
+        if isinstance(other, self.__class__):
+            return self.base == other.base and self.extras == other.extras
+        return False
+
+    # Needed for Python 2, which does not implement this by default
+    def __ne__(self, other):
+        # type: (Any) -> bool
+        return not self.__eq__(other)
+
     @property
     def name(self):
         # type: () -> str
@@ -404,6 +426,10 @@ class RequiresPythonCandidate(Candidate):
             version_info = sys.version_info[:3]
         self._version = Version(".".join(str(c) for c in version_info))
 
+    # We don't need to implement __eq__() and __ne__() since there is always
+    # only one RequiresPythonCandidate in a resolution, i.e. the host Python.
+    # The built-in object.__eq__() and object.__ne__() do exactly what we want.
+
     @property
     def name(self):
         # type: () -> str