This Bible contradiction is from the Skeptic's Annotated Bible.

Verses

She was his wife.

Genesis 25:1 View context

Then again Abraham took a wife, and her name [was] Keturah.

She was his concubine.

1 Chronicles 1:32 View context

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.