Flight plan having the aircraft identification YYY772 was already active in the ATC flight data processing system. This aircraft was not logged on with the ATS system.
Consequently the logon request was automatically accepted by the ATS system and automatically associated with the flight plan of YYY772.
Therefore the uplinked ATC clearances intended for YYY772 were actually received by YYY777.
Voice readback of the CPDLC instructions and other communications with the aircraft involved, triggered the recognition of the miss-match and the situation was clarified and resolved on the voice frequency.
When queried as to whether they received the CPDLC messages, the crew of YYY772 did not highlight the fact that they were not CPDLC connected at the time.
Aircraft Operators are Reminded to
Ensure that the correct aircraft identification (ICAO flight plan Item 7) is used for all airborne systems including CPDLC log-on.