For the NPE-240A models, a single external check valve should be effective when utilizing a single recirculation loop since it should prevent backward flow from the recirculation return line from flowing to faucets/appliances.
When the NPE-240A is not feeding an external recirculation loop (settings and 2-way valve set to Internal Circulation), then an external check valve will not be effective and the internal check valve will need to be repaired/replaced since a defective internal valve will allow cold incoming water to mix with the outgoing hot water internally, just above the WH's hot water connection.
Since the newer NPE-240A2 models have been redesigned with the pump located in an alternate position, I remain uncertain if a defective check valve will continue to permit cold water to mix with hot when configured for Internal Circulation.
Regardless of NPE-240A or NPE-240A2, if the home is equipped with multiple recirculation return loops, then each return will require its own check valve to prevent flow from each return from cross flowing to fixtures/appliances within an alternate recirculation loop.