ItemControl.selected = True fails if selected is already True

[mgedmin]

Given a form like

            <select multiple name="select-multiple">
              <option value="foo" />
              <option value="bar" />
              <option value="baz" />
            </select>

if you find the ItemControl for foo (call it item) and set item.selected = True twice in a row, zope.testbrowser ends up calling WebTest's MultipleSelect.value__set() with a list of values that contains duplicates, which in turn causes WebTest to raise

ValueError: Option(s) u'foo' not found (from u'foo', u'bar', u'baz')