Allow duplicate compound type aliases (#8788)

* fixes 8787

* Update CompoundTypeAliasTree.cs

---------

Co-authored-by: Ledjon Behluli <[email protected]>
Co-authored-by: Reuben Bond <[email protected]>

src/Orleans.Serialization/TypeSystem/CompoundTypeAliasTree.cs

@@ -93,7 +93,14 @@ private CompoundTypeAliasTree AddInternal(object key, Type? value)
         {
             if (value is not null && existing.Value is { } type && type != value)
             {
-                throw new ArgumentException("A key with this value already exists");
+                // When the same grain interface is used across multiple assemblies which don't have cross references,
+                // code-gen will generate code for both because it works in isolation, yet at startup they are combined.
+
+                // In this case, if the key is present, and the value is the same as the one being added,
+                // and due to them being logically the same, we can just return the existing CompoundTypeAliasTree.
+
+                // The first one is allowed to win in this case.
+                return existing;
             }
 
             existing.Value = value;
@@ -104,4 +111,4 @@ private CompoundTypeAliasTree AddInternal(object key, Type? value)
             return _children[key] = new CompoundTypeAliasTree(key, value);
         }
     }
-}
+}