Rashi on Genesis
קטורה KETURAH — This is Hagar. She was named Keturah because her deeds were as beautiful (sweet) as incense (Ketoreth) (Genesis Rabbah 61). And since she closed her 'opening,' as she did not mate with anyone from the time she separated from Avraham (Genesis Rabbah 61:4).
Rashbam on Genesis
קטורה, according to the plain meaning of the text this woman was not identical with Hagar.
Kli Yakar on Genesis
Avraham again took. The word vayosef — “again” — has connotations of continuing a previous relationship, hinting that this was the second time he married her. Keturah. See Rashi. Avraham named her in honor of the holy incense to make it known that she had repented fully after having lapsed into idolatry, so that now even her former deeds ascended to Heaven like the incense.