Hello James. The short answer as with many things is that it's open for judgement and depends on the preferences of your organization. However, I'd say it's fine.

RACI stands for Responsible, Accountable, Consulted, Informed.  Really it's best to start with the A. This is the person who has the "authority" and is therefore "accountable" and provides "approval".  There is only one A for every task, but you can have many Rs or Responsible people who will complete the work when a task or deliverable is large (like a Business Requirements Document that covers quite a lot).  Keep in mind that the RACI matrix can be applied to tasks or deliverables that are considerably different in size which is what leads to both your question and to your question.  If the task or the team is small, a manager, team lead, or a consultant may also complete the work. So they could be both the A and the R. But in my experience (and probably the preference of most organizations), it's usually best to have them be separate. 

One caveat is that I prefer the Accountable person to have the knowledge and skills to know that the task has been completed well.  So, if splitting the A and R roles means that the Accountable person is someone without the skills or knowledge necessary to assess completion and quality, then perhaps it's best to make a single person both Accountable and Responsible.

Here is a short article on the RACI Matrix