A potential PhD supervisor declined to accept me as a student; can I ask a second time? I found a relevant PhD supervisor who refused to supervise my PhD project by saying that he has no open position in his research group. Is it OK to mail him again and request to accept me as a student, because the work he is doing in his research group is extremely relevant for my research interests. I have received funding and just need space in a research group.
You can always ask.

However, be prepared that the answer may easily be still "no". Direct funding is by no means the only resource that an advisor has to pour into a student. There is also the time investment of actually advising a student, assorted costs for overheads and utilities, as well as presumably some money to fund conference trips. This isn't even to speak of simple inter-personal matters, such as that the professor may feel that you are not a good fit for the lab culture.