Store all duplicates of a node correctly upon their creation

Duplicates used to be stored in a dictionary with an entry being
"parent node": "duplicated node". On occasions where a single
parent node was duplicated more than once, the latest duplicated
 node erased the previous one(s), and these older ones were
"lost": after being created, there was no trace left of their
existence in the duplication operation. Undoing that duplication
operation was thus leaving these duplicated nodes out and not
removing them.

Duplicated nodes are now stored as "parent node": [list of
duplicated nodes] to keep track of all the created nodes,
effectively removing them upon an "undo".

tests/test_graph.py

@@ -266,14 +266,16 @@ def test_duplicate_nodes():
     # duplicate from n1
     nodes_to_duplicate, _ = g.dfsOnDiscover(startNodes=[n1], reverse=True, dependenciesOnly=True)
     nMap = g.duplicateNodes(srcNodes=nodes_to_duplicate)
-    for s, d in nMap.items():
-        assert s.nodeType == d.nodeType
+    for s, duplicated in nMap.items():
+        for d in duplicated:
+            assert s.nodeType == d.nodeType
 
-    # check number of duplicated nodes
-    assert len(nMap) == 3
+    # check number of duplicated nodes and that every parent node has been duplicated once
+    assert len(nMap) == 3 and all([len(nMap[i]) == 1 for i in nMap.keys()])
 
     # check connections
-    assert nMap[n1].input.getLinkParam() == n0.output
-    assert nMap[n2].input.getLinkParam() == nMap[n1].output
-    assert nMap[n3].input.getLinkParam() == nMap[n1].output
-    assert nMap[n3].input2.getLinkParam() == nMap[n2].output
+    # access directly index 0 because we know there is a single duplicate for each parent node
+    assert nMap[n1][0].input.getLinkParam() == n0.output
+    assert nMap[n2][0].input.getLinkParam() == nMap[n1][0].output
+    assert nMap[n3][0].input.getLinkParam() == nMap[n1][0].output
+    assert nMap[n3][0].input2.getLinkParam() == nMap[n2][0].output