First, I meant to say that Robert Duke died in 1784-5, not that he was born then!He would have been born about 1730-1736, to have married Priscilla Crenshaw in Lunenburg, VA before 1755 or so. The Mackey's, Duke's, McGarrah's, Hamilton's, Wade's, Seaborn's, Gaston's, Hicklin's, Perry's, and others, all came down through NC to SC, from VA, most likely from the Scots-Irish settlement in Augusta-Rockbridge-Botetourt counties, and the Crenshaw's (Thomas Crenshaw from Scotland) came to Isle of Wight Co., VA in the mid 1600's. A Daniel Mackey was also in early Isle of Wight Co., VA, but I don't see a connection with him, to John Mackey, or the Lancaster Co., SC Mackey's. All this above information is pretty much common factual knowledge.
I don't know about your later particular family, just that John Mackey was almost without doubt, the ancestor of probably all the lines of Mackey's in Lancaster/Fairfield, SC, and that they (the earlier Mackey's) were definitely married and or associated with these above families. John Mackey had Thomas, Solomon, William, George, and other sons that I don't know the names of.
I suggest that you go online to the SC Archives search page, where you can get accurate information. Also, the Lancaster, and Camden Co., Deed Books, and the Record Books of Lunenburg Co., VA. Lunenburg was full of Mackey/McKee/McKay names. I can also send you some documents, if you like. This is the best that I can do, for now, other than suggesting that you also check the Crenshaw, Duke, McGarra/McGarragh/McGarry, and Perry family GenForum boards.