Likutei Moharan 51:6/ליקוטי מוהר"ן נ״א:ו׳

16. Then [Rebbe Nachman] said: There is an inherent difficulty in this, something that cannot be understood. For if it is so, how will there be separate reward for each person commensurate with his level and to the degree of devotion and effort he exerted for God in this world. For certainly also at the very end not everyone will be equal. But since everything will be one, how is it possible that there will be a distinction between one person and another according to level. However, this involves a deep mystery which is impossible to understand, for the matter is beyond us.