Mercer Tilson, b. 1837, a Tilson descendant, wrote a book on the Tilsons. It was originally published in 1911, and on page 227 he states that Margaret Dungan is the daughter of Mary Frances Gaskins.
Margaret Dungan married William Tilson, son of Lemuel Tilson and Mary "Polly" Dungan. A book was also written a number of years ago on the Dungans. I believe it erroneously states that Margaret Dungan is the daughter of Rebecca Wilson, the second wife of John Robert Dungan. This error has been perpetuated for a number of year due to the Dungan book. Of course, we can't expect that the author was always 100 % correct.
George Dungan, b. abt. 1703, married Mary Hellings and had a son, Hellings Dungan, b. March 03, 1744/45, who married Sarah Ukn. George and Sarah had a daughter, Mary "Polly" Dungan, who married Lemuel Tilson, b. abt. 1773. Mary and Lemuel Tilson had a son named William Tilson, b. abt 1797. William Tilson married in abt. 1819-1820 to Margaret Dungan, b. abt. 1803 who is the daughter of John Robert Dungan (who also decends from George Dungan and Mary Hellings) and Mary Frances Gaskins. It is highly unlikely that Margaret is the daughter of Rebecca Wilson because Margaret's first child, John Tilson, was born about 1821. She had 9 children before she died sometime between 1846-1847 in Rocklane, Johnson Cty., Indiana. My grgrandfather, Byron Luther Tilson, was her last child, being born in 1844.
Rebecca Wilson Dungan was still having children when Margaret started having children in 1821. Rebecca did not marry John Robert Dungan till just before 1813, the year her first child was born. Also, Margaret named a daughter Mary, after her mother Mary Frances Gaskins. She named no daughters Rebecca, nor did any of her children use the name Rebecca for their children, although several used the name Mary.
Mary "Polly" and Margaret Dungan were first cousins, once removed and married a father and his son respectively.