Geometry Nodes: improve handling when the same socket is connected twice

The multi-input-socket cannot be connected to the same socket twice currently.
However, it is still possible to achieve this using an intermediate reroute node.

In this case the origin socket should be listed twice in the `linked_sockets_` list.
Higher level functions can still deduplicate the list of they want.

source/blender/nodes/intern/node_tree_ref.cc

@@ -181,7 +181,7 @@ void NodeTreeRef::find_origins_skipping_reroutes(InputSocketRef &socket,
       this->find_origins_skipping_reroutes(*direct_origin->node_->inputs_[0], r_origins);
     }
     else {
-      r_origins.append_non_duplicates(direct_origin);
+      r_origins.append(direct_origin);
     }
   }
 }