Vim filetype plugin executed twice when enter in insert mode?

Question

I am editing a file of filetype=help and want to do something when entering in insert mode. So I put a new ftplugin in ~/.vim/ftplugin/help.vim:

" ~/.vim/ftplugin/help.vim
echom 'execute help ftplugin'
augroup filetype_help
    autocmd! " or autocmd! InsertEnter <buffer>
    autocmd InsertEnter <buffer> echom 'enter insert mode'
augroup END

But it doesn't work when I firstly enter in insert mode. I also find that it will execute the plugin code in ~/.vim/ftplugin/help.vim when firstly entering in insert mode, which will show execute help ftplugin twice.

Maybe it only appears in some vim versions, and my system environment is:

• vim: VIM - Vi IMproved 7.4 (2013 Aug 10) containing patches: 1-207, 209-326.
• system: openSuse 42.3