Sarah was Abraham's wife. Hagar was a concubine given to Abraham by Sarah. After Sarah's death, Abraham took another "wife" according to Genesis 25:1. However in a later record she is called a concubine
Genesis 25:6 But unto the sons of the concubines, which Abraham had, Abraham gave gifts, and sent them away from Isaac his son, while he yet lived, eastward, unto the east country.
Genesis 25:1 Then again Abraham took a wife, and her name was Keturah.
Genesis 25:4 And the sons of Midian; Ephah, and Epher, and Hanoch, and Abida, and Eldaah. All these were the children of Keturah.
1 Chronicles 1:32 Now the sons of Keturah, Abraham's concubine: she bare Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah. And the sons of Jokshan; Sheba, and Dedan.
It would seem then that Abraham's sons by Keturah were not considered heirs so she was indeed a concubine as 1 Chronicles 1:32 has it. Genesis 25:1 is not incorrect because a concubine is a secondary wife. Genesis 25:6 seems to indicate that he had other concubines of lesser status than Keturah but we cannot be sure as we aren't told specifically who they were or what their names were.
Genesis 25:6 But to the sons of his concubines Abraham gave gifts, and while he was still living he sent them away from his son Isaac, eastward to the east country.
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