fixed bug where a variable appearing twice in an inputs list caused

theano/gradient.py

@@ -466,7 +466,7 @@ def _populate_var_to_node_to_idx(outputs):
     """
 
 
-    #var_to_node_to_idx[var][node] = i means node has var as input at position i
+    #var_to_node_to_idx[var][node] = [i,j] means node has var as input at positions i and j
     var_to_node_to_idx = {}
     #set of variables that have been added to their parents
     accounted_for = set([])
@@ -480,10 +480,17 @@ def _populate_var_to_node_to_idx(outputs):
         accounted_for.add(var)
         if var.owner is not None:
             node = var.owner
+            if node not in accounted_for:
+                accounted_for.add(node)
                 for i, ipt in enumerate(node.inputs):
                     if ipt not in var_to_node_to_idx:
                         var_to_node_to_idx[ipt] = {}
-                var_to_node_to_idx[ipt][node] = i
+                    node_to_idx = var_to_node_to_idx[ipt]
+                    if node not in node_to_idx:
+                        node_to_idx[node] = []
+                    idx = node_to_idx[node]
+                    assert i not in idx
+                    idx.append(i)
                     account_for(ipt)
 
     for output in outputs:
@@ -584,7 +591,13 @@ def _populate_grad_dict(var_to_node_to_idx,\
                 terms = []
                 node_to_idx = var_to_node_to_idx[var]
                 for node in node_to_idx:
-                    idx = node_to_idx[node]
+                    for idx in node_to_idx[node]:
+
+                        if hasattr(node.op, 'connection_pattern'):
+                            pattern = node.op.connection_pattern()
+                            if not pattern[idx]:
+                                continue
+
                         term = access_term_cache(node)[idx]
 
                         if not isinstance(term, gof.Variable):