Fix for python lattice generation

Author: giadarol

xtrack/_temp/python_lattice_writer/lattice_py_generation.py

@@ -36,15 +36,17 @@ def _elem_to_tokens(env, nn, formatter):
     if hasattr(ee, 'ksl'):
         fields += ['ksl']
 
-    if isinstance(ee, xt.RBend):
-        assert 'length_straight' in fields
-        fields = [ff for ff in fields if ff != 'length']
-
     tt = env[nn].get_table()
     for kk in tt.name:
+        if kk in fields:
+            continue
         if tt['expr', kk] is not None and tt['expr', kk] != 'None':
             fields.append(kk)
 
+    if isinstance(ee, xt.RBend):
+        assert 'length_straight' in fields
+        fields = [ff for ff in fields if ff != 'length']
+
     params = []
     for kk in fields:
         if kk == '__class__':