Allow multiple registrations of the same atom type if definitions identical (#4531)

* Allow multiple registrations of the same atom type if definitions identical

* Different short-circuiting logic

wrappers/python/openmm/app/forcefield.py

@@ -436,6 +436,10 @@ class ForceField(object):
         """Register a new atom type."""
         name = parameters['name']
         if name in self._atomTypes:
+            #  allow multiple registrations of the same atom type provided the definitions are identical
+            existing = self._atomTypes[name]
+            if existing.atomClass == parameters['class'] and existing.mass == float(parameters['mass']) and existing.element.symbol == parameters['element']:
+                return
             raise ValueError('Found multiple definitions for atom type: '+name)
         atomClass = parameters['class']
         mass = _convertParameterToNumber(parameters['mass'])