“ Text And to the sons of the concubines, that Abraham has, Abraham gave gifts; and he sent them away from upon Isaac, his son, in his being yet alive, towards the east, to eastern land.
Rashi The concubines
It is written defective, for there was only one concubine; she is Hagar, she is Keturah. Wives are with a ketubah, concubines are without a ketubah, as we say (in Sanhedrin page 21) concerning the wives and concubines of David. ”