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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def is_valid_sites(self):
 """ The sites are overdetermined """

        if 'site_model' in self.inputs and 'sites' in self.inputs:
            return False
        elif 'site_model' in self.inputs and self.sites:
            return False
        elif 'sites' in self.inputs and self.sites:
            return False
        elif self.sites and self.region and self.region_grid_spacing:
            return False
        else:
            return True