Remove duplicates in get_neighbors function (#591)

* Remove duplicates in get_neighbors function

* use set to remove duplicates

Author: lingxz

tensornetwork/network_operations.py

@@ -888,10 +888,14 @@ def get_neighbors(node: BaseNode) -> List[Node]:
     All of the neighboring edges that share an `Edge` with `node`.
   """
   neighbors = []
+  neighbors_set = set()
   for edge in node.edges:
     if not edge.is_dangling() and not edge.is_trace():
       if edge.node1 is node:
-        neighbors.append(edge.node2)
-      else:
+        if edge.node2 not in neighbors_set:
+          neighbors.append(edge.node2)
+          neighbors_set.add(edge.node2)
+      elif edge.node1 not in neighbors_set:
         neighbors.append(edge.node1)
+        neighbors_set.add(edge.node1)
   return neighbors

tensornetwork/tests/network_operations_test.py

@@ -513,3 +513,17 @@ def test_get_neighbors(backend):
     b[1] ^ b[2]
     result = tn.get_neighbors(b)
     assert result == [a, c]
+
+def test_get_neighbors_no_duplicates(backend):
+  with tn.DefaultBackend(backend):
+    a = tn.Node(np.ones((2, 2, 2)))
+    b = tn.Node(np.ones((2, 2, 2, 2, 2)))
+    c = tn.Node(np.ones((2, 2, 2)))
+    d = tn.Node(np.ones((2, 2)))
+    b[0] ^ a[0]
+    b[1] ^ a[1]
+    b[2] ^ c[0]
+    a[2] ^ d[1]
+    b[3] ^ b[4]
+    result = tn.get_neighbors(b)
+    assert result == [a, c]