Walking running distance double counting

I started going to gym and when I’m on the thread mill, I activate indoor walking workout on my watch and keep my phone on the machine, without any app activation. I walk for about 2km and then when I look at “walking distance” section in health app I see it registered as approx 4km.

when I look at the data details, I see same time stamp both for watch and iPhone record. I’m not sure why both of them are recording. Shouldn’t only one of them recording? I’m not giving any indication to my phone that I’m working out, even so, such cases like you go for outdoor walk, do both watch and phone records the walk and register twice as long? If so I didn’t realize it so far. Does anyone experience this?

Btw I see 2km when I look at the workouts under health app