Thanks for the link fieryjack.
I called Apple Support this morning who advised they are aware of the issue.
I informed Apple:
My old watch updated to version 6.3 and new watch to version 9.0. Both had repetitive charging complete sounds.
A hard reset fixed my old watch, however my new watch continued with the repetitive charge complete alert.
I reset the watch again and restored from a backup to find the issue continued.
Reluctantly I setup as new feeling pretty confident that fix the issue... but when the watch fully charged I woke up to - ding, ding, ding, ding, ding, ding...
Suggestions from Apple Support:
- Hard reset
- Setup watch as new
- Charge on silent until next update
I am keen to hear if anyone else has been able to resolve the issue a different way?