Fix duplicated exception chain output for exceptions without a traceback

AUTHORS

@@ -439,6 +439,7 @@ Sankt Petersbug
 Saravanan Padmanaban
 Sean Malloy
 Segev Finer
+SemTiOne
 Serhii Mozghovyi
 Seth Junot
 Shantanu Jain

changelog/8321.bugfix.rst

@@ -0,0 +1 @@
+Fixed exception chains being duplicated in the traceback output when an exception in the chain (``__cause__``/``__context__``) has no traceback of its own, and that exception is itself linked to further exceptions.

src/_pytest/_code/code.py

@@ -1226,7 +1226,9 @@ def repr_excinfo(self, excinfo: ExceptionInfo[BaseException]) -> ExceptionChainR
             else:
                 # Fallback to native repr if the exception doesn't have a traceback:
                 # ExceptionInfo objects require a full traceback to work.
-                reprtraceback = ReprTracebackNative(format_exception(type(e), e, None))
+                reprtraceback = ReprTracebackNative(
+                    format_exception(type(e), e, None, chain=False)
+                )
                 reprcrash = None
             repr_chain.append((reprtraceback, reprcrash, description))
 

testing/code/test_excinfo.py

@@ -1592,6 +1592,45 @@ def g():
             ]
         )
 
+    def test_exc_chain_repr_without_traceback_multiple_links(self) -> None:
+        """
+        Exceptions without a traceback that are themselves part of a longer
+        chain must not have their remaining chain printed twice: once by
+        Python's own ``traceback.format_exception`` (used as a fallback when
+        an exception has no ``__traceback__``) and once more by pytest's own
+        chain-walking loop (#8321).
+        """
+        exc1 = ValueError("abcd")
+        exc2 = IndexError("efgh")
+        exc3 = KeyError("ijkl")
+        exc4 = RuntimeError("mnop")
+        exc1.__cause__ = exc2
+        exc2.__context__ = exc3
+        exc3.__cause__ = exc4
+
+        try:
+            raise exc1
+        except ValueError:
+            excinfo = ExceptionInfo.from_current()
+
+        r = excinfo.getrepr()
+        file = io.StringIO()
+        tw = TerminalWriter(file=file)
+        tw.hasmarkup = False
+        r.toterminal(tw)
+
+        output = file.getvalue()
+        for message in (
+            "ValueError: abcd",
+            "IndexError: efgh",
+            "KeyError: 'ijkl'",
+            "RuntimeError: mnop",
+        ):
+            assert output.count(message) == 1, (
+                f"{message!r} should appear exactly once in the output, "
+                f"got {output.count(message)} occurrences:\n{output}"
+            )
+
     def test_exc_chain_repr_cycle(self, importasmod, tw_mock):
         __tracebackhide__ = True
         mod = importasmod(