In the Bible we are told that God created man in His own image [Genesis 1:27] and later walks in the garden [Genesis 3:8]. He speaks to Moses face to face as with a friend [Exodus 33:11], wrestles physically with Jacob [Genesis 32:22-32], and appears to Ezekiel as a man [Ezekiel 1:26]. These manifestations of God are called theophanies. There are many more manifestations of God in the Bible (such as Exod 3:3–12, Exod 13:21–22, Exodus 19:16–25, Psalm 18:8–16) but in these specifically, God appears in human form.
An incarnation is simply a person who embodies a deity, spirit, or abstract quality. Certainly these manifestations qualify. Wasn’t this God the Father made flesh? He was not human, per se, He merely assumed the form of a human being. Even so, weren’t these literal incarnations? “Person” does not necessarily mean “human”. Remember we call the triune godhead “God in three persons“.
Jesus, on the other hand, was both fully human and fully divine. He was the only truly human incarnation of God. Jesus often called himself the “Son of Man”. Another translation of this is the “Human One”.
And so, while there may have been other personal incarnations of God, they were definitely not true human beings like Jesus. Inasmuch as this is concerned, Jesus was unique.