It is unique and in fact contrived when translated as such in Luke 1:28. "Full of grace" is translated only once in the scripture and that applied to Jesus in John 1:14, "And the Word was made flesh, and dwelt among us, (and we beheld his glory, the glory as of the only begotten of the Father,) full of grace and truth." The Greek is pleres charis.
"pleres charis" indeed is mentioned several times, but "kecharitomenos/e" is a word formation uniquely describing Mary in the New Testament. The criticism that "gratia plena" perhaps should not be used for both epithets is valid, but certainly the milquetoast "most favored" is no better, if only because it hides the aorist which is theologically significant. If anything, when describing a temporal bestowal of grace rather than a preexisting fullness of grace, "gratia plena" should not have been used.