From my research, it looks like Josiah Duer (b. June 10, 1796 in Bucks Co., PA, d. April 25, 1860 in Miami Co., OH was the first Duer to move to Miami County, Ohio.He and Mary Suber Duer had 8 children:
Charles, b. 1821 in Bucks Co., PA Joseph Suber Duer, B. 1822 in Bucks Co., PA John Hollingshead Duer, b. 1824 in Bucks C., PA Albert Duer, b. 1826 in Bucks Co., PA Sarah Ann Duer, b. 1829 in Miami Co., OH Horatio Duer, b. 1831 in ? Edwin Augustus Duer, b. 1831 in ? Lidia Ann Duer, b. 1834 in OH
So, it looks like Duers came to Miami County, OH between 1826 and 1829.
Some of these children stayed in Miami, County and some ended up back in Bucks County
Josiah's genealogy can be traced back 6 generations (two of them in Devonshire, England).Thomas Duer came from Devonshire to Bucks Co., PA in 1683 and was Josiah's Great Grandfather.Most of this is available (with some errors) on this website.
There are other related Duers that moved from Bucks Co. PA and settled in Trumbal Co., OH as well.