See also s.vv. rb, rbrbtn, rbtn.
The form is vocalized as "rəraḇtān" in Isaiah as opposed to the usage in Ezekiel; however, that is surely due to the required vocalization of the regular variant where the initial resh is read as a daleth, i.e. "d_" prep. We assume this is a legitimate development from the expanded form of "rb" adj., i.e. "rbrb".