"duplicate tag" errors when running :helptags

Vundle reports errors when generating helptags after installing vim-autocomplpop. The output of running `:helptags` is included below. It appears to simply be caused by a couple of typos in the help file.

```
E154: Duplicate tag "g:acp_refeed_after_every_char" in file /[...]/bundle/vim-autocomplpop/doc//acp.txt
E154: Duplicate tag "popupmenu-keys" in file /[...]/bundle/vim-autocomplpop/doc//acp.txt
Press ENTER or type command to continue
```