Nothing build in, but I think something similar is on the roadmap (longer term). What you can do is create a data action to search the queue for interactions and search for the phone number in the result. Then if it is found, tell the caller they already have a callback in queue and that will be replied to before them waiting in line and also, don't offer the callback if there is one in queue already.
------------------------------
Robert Wakefield-Carl
Avtex Solutions, LLC
Contact Center Innovation Architect
robertwc@avtex.comhttps://www.Avtex.comhttps://RobertWC.Blogspot.com------------------------------