According to Genesis 25:1, Keturah is described as being Abraham’s wife:
“Abraham married another wife named Keturah.”
The word used for wife in the Hebrew is 
(‘ishshah) (Strong’s No. 802)[1] and The Brown-Driver-Briggs Hebrew and English Lexicon (2001) defines the meaning of the word as “woman, wife, female”[2].
But according to I Chronicles 1:32, we are told that Keturah was Abraham’s concubine:
“Sons of Keturah, Abraham’s concubine: Zimran, Jokshan….”
The Hebrew used here according to the BDB lexicon is either 
(piylegesh) or 
(piyegesh) (Strong’s No. 6370)[3] and the lexicon defines the meaning as “concubine”[4].
It is not possible for Keturah to be both a wife and concubine. The difference is without a doubt truly glaring, that one wonders what the missionary excuse will be for the above contradiction.
References
[1] James Strong, The New Strong’s Exhaustive Concordance of the Bible (Thomas Nelson, 1996)
[2] F Brown, S. Driver & C. Briggs, The Brown-Driver-Briggs Hebrew and English Lexicon (2001), p. 61
[3] James Strong, op. cit.
[4] F. Brown, S. Driver & C. Briggs, 0p. cit., p. 811
