I think it is most likely a filled MM due to a die failure. VV does not have an RPM for this date & MM. http://www.varietyvista.com/12 Kennedy Halves/RPM Listings.htm
To me, the D mint mark looks like a normal D, with another D on top of it - slightly tilted to the N.W.
Looks like a typical filled die that may have taken a small hit on the bottom. Not seeing anything that would indicate a new RPM.