Avoid duplicate materials written to XML (#3536)

Co-authored-by: Paul Romano <paul.k.romano@gmail.com>

Author: RemDelaporteMathurin

openmc/material.py

@@ -1927,7 +1927,7 @@ def _write_xml(self, file, header=True, level=0, spaces_per_level=2,
             file.write(ET.tostring(element, encoding="unicode"))
 
         # Write the <material> elements.
-        for material in sorted(self, key=lambda x: x.id):
+        for material in sorted(set(self), key=lambda x: x.id):
             element = material.to_xml_element(nuclides_to_ignore=nuclides_to_ignore)
             clean_indentation(element, level=level+1)
             element.tail = element.tail.strip(' ')

tests/unit_tests/test_materials.py

@@ -61,3 +61,19 @@ def test_materials_deplete():
     # Ni59 is one of the main activation product of Ni60 in the first irradiation
     # step. It then decays in the second cooling step (flux = 0)
     assert Ni59_mat_1_step_1 > 0.0 and Ni59_mat_1_step_1 > Ni59_mat_1_step_2
+
+
+def test_export_duplicate_materials_to_xml(run_in_tmpdir):
+    """
+    Test exporting Materials to xml with a duplicate and checking that only
+    unique entities are exported.
+    """
+    my_mat = openmc.Material(name="my_mat")
+    my_mat2 = openmc.Material(name="my_mat2")
+
+    materials = openmc.Materials([my_mat, my_mat2, my_mat])
+
+    materials.export_to_xml("materials.xml")
+
+    materials_in = openmc.Materials.from_xml("materials.xml")
+    assert len(materials_in) == 2