I did a search and came up with this: LLANFAIRPWLLGWYNGYLLGOGERYCHWYRNDROBWLLLLANTYSILIOGOGOGOCH, which is the name of a Welsh town. Since your example of “runner” has the “n” and the “r” as being repeated, my example has “L”, “A”, “N”, “I”, “R”, “W”, “Y”, “G”, “O” and “R” 10 repeated letters. Oddly, it only has 5 letters that AREN’T repeated.