Maybe it has something to do with professions. What profession is he? and which are you.
My wife plays a Ranger she get double herb nodes alot.\
I play a Necro and an Elementalist I do not get any doubles.
By doubles I mean plant is there for each of us. I pick mine it is gone. She picks hers and there is another there.
I do not believe it is MF as I have been noticing this occurrence a lot lately. My Ranger has 140%MF and gets a lot of double harvests and I started playing my Ele a lot more recently and she is seeing quite a few double harvests (and she has no MF). I have noticed that the frequency of this occurring has increased over the last few weeks also.
One thing that has effected harvesting recently is the WvW bonuses are active. So as your world earns points your bonuses (that you can't really see unless you hit "B" and look at the what you are getting overall) you will have a higher and higher chance of double harvesting and also getting extras like gems or food add ins. I'm not sure when the bonuses started working, but its been about 2 or 3 weeks.