It depends. Is the case being published in a journal? Or are you sending it to a conference? If you are just presenting the case (and it won't be a published article), that almost never precludes later publication so feel free to include it in a case series. Otherwise read on.
My gut says that this would raise some eyebrows. The biggest issue I see is that a case report and a case series are essentially the same from a birds-eye view.
Think of it this way, a case series is just that - a series of individual case reports that have been compiled for one reason or another. You're generally not analyzing data, there is no experiment taking place, your conclusions are almost always limited and not generalizable. The value of either individual reports or series comes from the fact that those cases are unique and novel. Assuming you're working with 3-5 cases, which would be the usual limit for a case series, having one of those cases previously published is essentially double dipping. There is nothing stopping you from referencing that other case but, by virtue of having been previously published, it isn't unique or novel.
I think there are other issues too. Copyright would be a concern, depending on where you publish the original case. You will also be asked to certify that your paper has not been published (in full or in part) elsewhere. Then of course there is the question of ethics. Are you introducing some bias (however limited it may be) into the literature by publishing the same case multiple times? How will people know that these cases are one and the same and not two unique examples of a particular disease/treatment/presentation. This also opens you up to accusations of "salami slicing". If there is something of value to be said about a particular case, why not include it in the initial case report? Other than bumping up your publication numbers, what is the benefit from addressing the same patient in two separate publications?
I wouldn't open this can of worms, but you can always include it and just state that the case has been previously published. From there it's up to the editor at whatever journal you send it to.
