connection fix from @wilecoyote2015

Author: jchanvfx

NodeGraphQt/base/graph.py

@@ -1293,6 +1293,7 @@ def _deserialize(self, data, relative_pos=False, pos=None):
 
         # build the nodes.
         nodes = {}
+        new_node_connections = {}
         for n_id, n_data in data.get('nodes', {}).items():
             identifier = n_data['type_']
             node = self._node_factory.create_node_instance(identifier)
@@ -1318,21 +1319,27 @@ def _deserialize(self, data, relative_pos=False, pos=None):
         # build the connections.
         for connection in data.get('connections', []):
             nid, pname = connection.get('in', ('', ''))
-            in_node = nodes.get(nid)
+            in_node = nodes.get(nid) or self.get_node_by_id(nid)
             if not in_node:
                 continue
             in_port = in_node.inputs().get(pname) if in_node else None
 
             nid, pname = connection.get('out', ('', ''))
-            out_node = nodes.get(nid)
+            out_node = nodes.get(nid) or self.get_node_by_id(nid)
             if not out_node:
                 continue
             out_port = out_node.outputs().get(pname) if out_node else None
 
             if in_port and out_port:
-                self._undo_stack.push(PortConnectedCmd(in_port, out_port))
-
-        node_objs = list(nodes.values())
+                # only connect if input port is not connected yet or input port
+                # can have multiple connections.
+                # important when duplicating nodes.
+                allow_connection = any([not in_port.model.connected_ports,
+                                        in_port.model.multi_connection])
+                if allow_connection:
+                    self._undo_stack.push(PortConnectedCmd(in_port, out_port))
+
+        node_objs = nodes.values()
         if relative_pos:
             self._viewer.move_nodes([n.view for n in node_objs])
             [setattr(n.model, 'pos', n.view.xy_pos) for n in node_objs]