diff --git a/src/poetry/core/version/markers.py b/src/poetry/core/version/markers.py
index ddef93703..f620f99c9 100644
--- a/src/poetry/core/version/markers.py
+++ b/src/poetry/core/version/markers.py
@@ -370,11 +370,14 @@ def _flatten_markers(
 
     for marker in markers:
         if isinstance(marker, flatten_class):
-            flattened += _flatten_markers(
+            for marker in _flatten_markers(
                 marker.markers,  # type: ignore[attr-defined]
                 flatten_class,
-            )
-        else:
+            ):
+                if marker not in flattened:
+                    flattened.append(marker)
+
+        elif marker not in flattened:
             flattened.append(marker)
 
     return flattened