[Backport maintenance/2.17.x] Fix Pyreverse duplicate arrows bug

doc/whatsnew/fragments/8522.bugfix

@@ -0,0 +1,3 @@
+Don't show arrows more than once in Pyreverse diagrams.
+
+Closes #8522

pylint/pyreverse/diagrams.py

@@ -230,9 +230,13 @@ def extract_relationships(self) -> None:
                         value, obj, name, "aggregation"
                     )
 
-            for name, values in list(node.associations_type.items()) + list(
-                node.locals_type.items()
-            ):
+            associations = node.associations_type.copy()
+
+            for name, values in node.locals_type.items():
+                if name not in associations:
+                    associations[name] = values
+
+            for name, values in associations.items():
                 for value in values:
                     self.assign_association_relationship(
                         value, obj, name, "association"

tests/pyreverse/functional/class_diagrams/attributes/duplicates.mmd

@@ -14,4 +14,3 @@ classDiagram
     example2 : int
   }
   A --* DuplicateArrows : a
-  A --* DuplicateArrows : a

tests/pyreverse/functional/class_diagrams/attributes/duplicates.py

@@ -8,7 +8,7 @@ def __init__(self):
         self.example2 = 2
 
 
-# OPEN BUG: https://github.com/pylint-dev/pylint/issues/8522
+# Test for https://github.com/pylint-dev/pylint/issues/8522
 class A:
     pass