וַיִּקְרָ֨א אֵלָ֜יו מַלְאַ֤ךְ יְהוָה֙ מִן־הַשָּׁמַ֔יִם וַיֹּ֖אמֶר אַבְרָהָ֣ם ׀ אַבְרָהָ֑ם וַיֹּ֖אמֶר הִנֵּֽנִי׃
האָט אַ מלאך פֿון גאָט גערופֿן צו אים פֿון הימל, און האָט געזאָגט: אבֿרהם! אבֿרהם! האָט ער געענטפֿערט: דאָ בין איך.
Rashi
אברהם אברהם ABRAHAM, ABRAHAM — The repetition of his name is an expression of affection (Genesis Rabbah 56:7).
Bekhor Shor
And an angel of God called to him from the Heavens: And even though he was speaking in the [manner] of the Divine Presence - as he said, "and you have not withheld your son [...] from Me" (Genesis 22:12), it is since in every place when the messenger of the Omnipresent would speak, it is as if the Holy One would speak. And the end shows that it was an angel - as the second time, he said (Genesis 22:16), "'I have sworn by Myself,' declares the Lord."
Chizkuni
אברהם, אברהם! “Avraham, Avraham!” The repetition of Avraham’s name is to indicate that the call was of an urgent nature. This is a formulation also known in other languages.