It sounds like there are two issues here, one about them having everything double, and the second that they just seem to have a lot in the first place.
I am surprised about the responses about being at home at the nrp. I find it incredibly sad that some step-mum would consider it wrong that their step-children should feel at home at their father's home. It is no surprise that one of the common complaint of step-children is that they don't feel welcome when they go there.
I didn't get along with my step-mum when growing up, but I did feel at home there. That's because they made an effort to make it so. I had a nice bedroom, with a nice bed, chose the wallpaper (which I loved), the bed, the wardrobe. I felt as much in my sanctuary in that room then I did in my room at my mum's.
However, that doesn't mean that I had all the luxuries available at both houses. Almost all the toys and clothing I had there were what I got from my dad and his family for Christmas and birthdays. If they bought any other item of clothing at any other time, it was supermarket things.
If money is an issue, then it is about limiting the non essential items, but it is nice for the children to have some things staying there to avoid that feeling of 'visiting', which I find very depersonalising.