I’ve had an idea fermenting for a while, and the idea has grown to a few possible instalments (I wouldn’t really say chapters, as they would all be fairly standalone stories), however I feel the story line will be very ‘same-y’ to my others stories, but just set in a different environment, so I’m not entirely sure that I should write them 
So my question is, do people find that an issue?
In my experience, porn is one of those creative areas where writing repetitive plots/scenarios/characters gets rewarded. If you’re too obviously plagiarizing yourself, that’s likely to get some pushback, but if the whole point is to get people off, what worked before is likely to work again.
That being said, as a writer I don’t like repeating myself too much; half the fun for me is the challenge of trying to describe something new and original. However, that often leads to me writing stuff way too obscure for the site, so it has its risks. If you feel like you have a particular style and a set of genre conventions you like to return to, I imagine your fans will respond very positively if you give them another example of those things.
I do this All. The Freakin’. Time! 
In my collection of unfinished/unpublished stories, I’ve even got one folder titled “Next Door Neighbour Variants” because I’ve written three different stories stemming from the same underlying idea, and even glancing at those, I don’t see the initial story, so either I scrapped that or it got modified enough that I don’t recognized it at a glance.
Even right now, as I work on a chapter that’s about ten ahead of the current one for Web of Trust (working on what I’m supposed to is an issue for me 
), I realize that I’ve used several of the same themes I’ve used in other stories, but spun them into a unique chapter in its own right. As sqneemie says, though, I don’t think that’s a bad thing as long as you don’t overdo it.
I don’t think I’d object. If I hypotheically reached some sort of ‘burnout level’ on a type of story that I otherwise liked, I’d simply set aside any new ones for a while and then come back to reading them later. No big deal.
And from a creative point of view, if you think your stories may be getting same-y, may I suggest adding one or two minor twists (in plot, or writing style, or character archetypes, or whatever…). Not enough to alienate any fans who are really keen on your work, but enough to make the story feel fresh.