Margaret Kennedy of Girvanmainly - Same Person as Lady Margaret Stewart?

I almost have my ancestry traced to Edward III, but this bit is confusing me. I have:
John MacDowell, b. 1575 to @Uchtred MacDowall of Garthland and this woman. However, she appears to be the same person as Lady Margaret Stewart, Uchtred's second wife. Is @John MacDowell the son of Lady Margaret, Margaret Kennedy, or are they the same person?