STM32F207 FreeRTOS-Plus-TCP loosing pings

Absolutely! I just wonder how a device should be able to detect IP address conflicts for its own IP address if the MAC filter prevents it from even seeing those weird packets (directed, by definition, to a different device)?

Of course, if the device sees multiple ARP mappings for a REMOTE IP address, that can and should be detected (should also be fairly straightforward to implement via the ARP cache).