Genesis 25:1 says Abraham took another wife, Keturah, but 1 Chronicles 1:32 calls her Abraham’s concubine. The argument notes that a wife and a concubine are not the same thing, so the two books describe her status differently.
Abraham took another wife, whose name was Keturah.
Genesis 25:1
The sons of Keturah, Abraham’s concubine: she bore Zimran, Jokshan, Medan, Midian, Ishbak, and Shuah. The sons of Jokshan: Sheba and Dedan.
1 Chronicles 1:32
Source: see Credits and references (no. 24, p. 23).