I can't tell you for sure why you got those duplicates (multiple sourcing?), but the canonical way to prevent this is clearing the augroup
first:
augroup asciidocFileType
autocmd!
autocmd FileType asciidoc :call AsciidocFocusLost()
augroup END
Since you only have one definition, you can also do this in one command:
augroup asciidocFileType
autocmd! FileType asciidoc :call AsciidocFocusLost()
augroup END