Guys, I do not believe Apple Watch and iPhone are double counting steps. This has been a source of concern for me as well. I haven't calculated the entire data for a specific day, but for instance, take a look at 3 simple calculation examples from my personal data. I chose 3 days at random:
Sept. 13, 2017
Apple Watch steps: 18,895
Apple iPhone steps: 1,761
Combined total: 20,656
Reported Health total: 19,045
Aug. 7, 2017
Apple Watch steps: 10,100
Apple iPhone steps: 9,453
Combined total: 19,553
Reported Health total: 10,366
Jul. 11, 2017
Apple Watch steps: 7,611
Apple iPhone steps: 2,282
Combined total: 9,893
Reported Health total: 7,924
You can try to do this on your phone. I think Apple has worked out a system to not double-count steps maybe when both devices are within bluetooth connectivity range of each other. So if you set your Apple Watch as the top priority source and it is within bluetooth range of your iPhone, then I believe although steps will be tracked on your iPhone as well, it will not be considered in the day's aggregate data.
Stephen