diff --git a/Lib/inspect.py b/Lib/inspect.py
index c2a32fe..9336943 100644
--- a/Lib/inspect.py
+++ b/Lib/inspect.py
@@ -728,7 +728,8 @@
             for parent in c.__bases__:
                 if not parent in children:
                     children[parent] = []
-                children[parent].append(c)
+                if c not in children[parent]:
+                    children[parent].append(c)
                 if unique and parent in classes: break
         elif c not in roots:
             roots.append(c)
diff --git a/Lib/test/inspect_fodder.py b/Lib/test/inspect_fodder.py
index afde2e2..5c87ae6 100644
--- a/Lib/test/inspect_fodder.py
+++ b/Lib/test/inspect_fodder.py
@@ -49,6 +49,8 @@
 class MalodorousPervert(StupidGit):
     pass
 
+Tit = MalodorousPervert
+
 class ParrotDroppings:
     pass
 
diff --git a/Lib/test/test_inspect.py b/Lib/test/test_inspect.py
index 04dcfe9..4130cd0 100644
--- a/Lib/test/test_inspect.py
+++ b/Lib/test/test_inspect.py
@@ -220,8 +220,23 @@
                          [('FesteringGob', mod.FesteringGob),
                           ('MalodorousPervert', mod.MalodorousPervert),
                           ('ParrotDroppings', mod.ParrotDroppings),
-                          ('StupidGit', mod.StupidGit)])
-        tree = inspect.getclasstree([cls[1] for cls in classes], 1)
+                          ('StupidGit', mod.StupidGit),
+                          ('Tit', mod.MalodorousPervert),
+                         ])
+        tree = inspect.getclasstree([cls[1] for cls in classes])
+        self.assertEqual(tree,
+                         [(mod.ParrotDroppings, ()),
+                          [(mod.FesteringGob, (mod.MalodorousPervert,
+                                                  mod.ParrotDroppings))
+                           ],
+                          (mod.StupidGit, ()),
+                          [(mod.MalodorousPervert, (mod.StupidGit,)),
+                           [(mod.FesteringGob, (mod.MalodorousPervert,
+                                                   mod.ParrotDroppings))
+                            ]
+                           ]
+                          ])
+        tree = inspect.getclasstree([cls[1] for cls in classes], True)
         self.assertEqual(tree,
                          [(mod.ParrotDroppings, ()),
                           (mod.StupidGit, ()),
diff --git a/Misc/NEWS b/Misc/NEWS
index 14201ca..3bbced3 100644
--- a/Misc/NEWS
+++ b/Misc/NEWS
@@ -32,6 +32,9 @@
 Library
 -------
 
+- Issue #18830: inspect.getclasstree() no more produces duplicated entries even
+  when input list contains duplicates.
+
 - Issue #18909: Fix _tkinter.tkapp.interpaddr() on Windows 64-bit, don't cast
   64-bit pointer to long (32 bits).