I'd like to better understand the interaction of MFP and my Charge HR fitbit

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Hi, I was on MFP for a while did well, went off it and the weight crept back. Now I got back on and got a fitbit, which I have synced with MFP. I need some plain language answers to my questions such as: I like to log exercise other than walking on MFP. But if I log two hours of gardening, Fitbit notices I'm walking around, though it requires other movement too. So, if I log the gardening on MFP am I getting credit for burning too many calories, because Fitbit is also doing an adjustment, just because I am wearing it. I am so confused. I bike a lot and that seems clear to me i.e. that I can log it on MFP and there will be no double counting, because Fitbit doesn't notice my biking. Help! Dead simple language please!