These are the names of Esau's sons; Eliphaz, Eliphaz... And Timna was concubine to Eliphaz.... Genesis 36:10-12
The sons of Esau; Eliphaz.....The sons of Eliphaz ... Timnah.... 1 Chronicles 1:35-36
These are the sons of Seir the Horite, who inhabited the land; Lotan ... and Lotan's sister was Timna. Genesis 36:20-22
The sons of Seir; Lotan ... and Timna was Lotan's sister. 1 Chronicles 1:38-39
Both of these verses come from lists of Esau's descendants, so it seems likely Timnah that refers to the same person.
Credit: All That's Wrong with the Bible: Contradictions, Absurdities, and More by Johan David Conner
I found a third Timna in Genesis 36! This Timna was Lotan's sister (v.22), Lotan being a son of Seir (v.20). So, Timna was a daughter of Seir. This Timna would've been completely unrelated to Eliphaz and would've been born a few generations before him.
There are also verses in 1 Chronicles chapter 1 that say the same thing. (v.38/39)
Thanks again, SandColoredSky.
I've added these verses to the contradiction.