diff --git a/HISTORY.md b/HISTORY.md
index c0d5be50..d0643cae 100644
--- a/HISTORY.md
+++ b/HISTORY.md
@@ -31,6 +31,8 @@ Our backwards-compatibility policy can be found [here](https://github.com/python
   ([#707](https://github.com/python-attrs/cattrs/issues/707) [#708](https://github.com/python-attrs/cattrs/pull/708))
 - Enum handling has been optimized by switching to hook factories, improving performance especially for plain enums.
   ([#705](https://github.com/python-attrs/cattrs/pull/705))
+- Fix `include_subclasses` when used with `configure_tagged_union` and classes using diamond inheritance.
+  ([#685](https://github.com/python-attrs/cattrs/issues/685) [#713](https://github.com/python-attrs/cattrs/pull/713))
 
 ## 25.3.0 (2025-10-07)
 
diff --git a/src/cattrs/strategies/_subclasses.py b/src/cattrs/strategies/_subclasses.py
index 41f812aa..695b0115 100644
--- a/src/cattrs/strategies/_subclasses.py
+++ b/src/cattrs/strategies/_subclasses.py
@@ -15,9 +15,13 @@
 def _make_subclasses_tree(cl: type) -> list[type]:
     # get class origin for accessing subclasses (see #648 for more info)
     cls_origin = typing.get_origin(cl) or cl
-    return [cl] + [
-        sscl for scl in subclasses(cls_origin) for sscl in _make_subclasses_tree(scl)
-    ]
+
+    # Use a dict to deduplicate and keep insertion order.
+    seen = {cl: None}
+    for scl in subclasses(cls_origin):
+        for sscl in _make_subclasses_tree(scl):
+            seen[sscl] = None
+    return list(seen)
 
 
 def _has_subclasses(cl: type, given_subclasses: tuple[type, ...]) -> bool:
diff --git a/tests/strategies/test_include_subclasses.py b/tests/strategies/test_include_subclasses.py
index e45c7fe0..d485c18e 100644
--- a/tests/strategies/test_include_subclasses.py
+++ b/tests/strategies/test_include_subclasses.py
@@ -509,3 +509,30 @@ class ChildDC1(ParentDC):
     include_subclasses(ParentDC, genconverter)
 
     assert genconverter.structure({"a": 1, "b": "a"}, ParentDC) == ChildDC1(1, "a")
+
+
+def test_diamond_inheritance(genconverter: Converter):
+    """Diamond inheritance is handled correctly (issue #685)."""
+
+    @define
+    class Base:
+        pass
+
+    @define
+    class Mid1(Base):
+        pass
+
+    @define
+    class Mid2(Base):
+        pass
+
+    @define
+    class Sub(Mid1, Mid2):
+        pass
+
+    # This should not raise an error
+    include_subclasses(Base, genconverter, union_strategy=configure_tagged_union)
+
+    assert genconverter.structure({"_type": "Sub"}, Base) == Sub()
+    assert genconverter.structure({"_type": "Mid1"}, Base) == Mid1()
+    assert genconverter.structure({"_type": "Mid2"}, Base) == Mid2()