More than eight repeated letters...
Well you did ask!
There are 9 A's in AQUAETHYLENEDIAMINETETRAACETATOMANGANATE (An inorganic compound)
There are 9 E's in DIHYDROXYBENZENEHEXAMETHYLENETETRAMINE (A chemical compound)
There are 9 I's in FLOCCINAUCINIHILIPILIFICATION (An estimation of something as worthless)
There are 11 L's LLANFAIRPWLLGWYNGYLLGOGERYCHWYRNDROBWLLLLANTYSILIOGOGOGOCH (A Welsh town)
There are 9 M's in DIMETHYLAMINOMETHYLENEAMINOMETHYLENEDIMETHYLAMMONIUM (A chemical compound)
There are 9 O's in PNEUMONOULTRAMICROSCOPICSILICOVOLCANOCONIOSIS (A lung disease caused by the inhalation of very fine silica dust)
There are 9 S's in POSSESSIONLESSNESSES
There are 9 U's in HUMUHUMUNUKUNUKUAPUAA (A triggerfish),
I like the letter Z so ZENZIZENZIZENZIC (The eighth power of a number) gets an honourable mention even though there are only 6 Z's.
(Thanks to Google, various letters sites and a lot of ground work by me!)