What are the implications of "Ki Avadai Heim"?
Rashi: It is a reminder that Hashem's document (that we are His Avadim) 1 precedes the document of sale that the Eved Ivri gave to his master.
Seforno: Even though the Eved acquired a master for himself, and therefore deserves to perform whatever tasks are placed upon him, due to the fact that he is Hashem's Eved, he does not have the authority to sell himself as an Eved, unconditionally.
See Torah Temimah, note 321.
Having already said (in Pasuk 39)"Lo Sa'avod bo Avodas Aved", why does it repeat "Lo Yimachru Mimkeres Aved"?