diff --git a/chatmastermind/chat.py b/chatmastermind/chat.py
index 41d12b9..fdfc3d3 100644
--- a/chatmastermind/chat.py
+++ b/chatmastermind/chat.py
@@ -119,6 +119,25 @@ class Chat:
 
     messages: list[Message]
 
+    def __post_init__(self) -> None:
+        self.validate()
+
+    def validate(self) -> None:
+        """
+        Validate this Chat instance.
+        """
+        def msg_paths(stem: str) -> list[str]:
+            return [str(fp) for fp in file_paths if fp.stem == stem]
+        file_paths: set[Path] = {m.file_path for m in self.messages if m.file_path is not None}
+        file_stems = [m.file_path.stem for m in self.messages if m.file_path is not None]
+        error = False
+        for fp in file_paths:
+            if file_stems.count(fp.stem) > 1:
+                print(f"ERROR: Found multiple copies of message '{fp.stem}': {msg_paths(fp.stem)}")
+                error = True
+        if error:
+            raise ChatError("Validation failed")
+
     def msg_name_matches(self, file_path: Path, name: str) -> bool:
         """
         Return True if the given name matches the given file_path.
@@ -276,6 +295,7 @@ class ChatDB(Chat):
         # make all paths absolute
         self.cache_path = self.cache_path.absolute()
         self.db_path = self.db_path.absolute()
+        self.validate()
 
     @classmethod
     def from_dir(cls: Type[ChatDBInst],
diff --git a/tests/test_chat.py b/tests/test_chat.py
index 4962688..18cc4ef 100644
--- a/tests/test_chat.py
+++ b/tests/test_chat.py
@@ -198,6 +198,17 @@ class TestChatDB(unittest.TestCase):
         self.cache_path.cleanup()
         pass
 
+    def test_validate(self) -> None:
+        duplicate_message = Message(Question('Question 4'),
+                                    Answer('Answer 4'),
+                                    {Tag('tag4')},
+                                    file_path=pathlib.Path('0004.txt'))
+        duplicate_message.to_file(pathlib.Path(self.db_path.name, '0004.txt'))
+        with self.assertRaises(ChatError) as cm:
+            ChatDB.from_dir(pathlib.Path(self.cache_path.name),
+                            pathlib.Path(self.db_path.name))
+        self.assertEqual(str(cm.exception), "Validation failed")
+
     def test_from_dir(self) -> None:
         chat_db = ChatDB.from_dir(pathlib.Path(self.cache_path.name),
                                   pathlib.Path(self.db_path.name))