Duplicated and appended URL

A lot of configuration options and the server's redirect configuration can come into play when using redirect() or site_url() (used by redirect() when the first parameter for redirect() doesn't include the protocol). The most obvious configuration options to check are 'base_url', 'index_page', and 'url_suffix'.

Additionally, if you've made any changes to the system files, it would be a good idea to undo those changes/replace them with the original files to see if the problem is caused by something that was changed. In general, changing a system file is a last resort, especially when things work fine in development (which indicates a server configuration issue or something specific to the differences between the server and development machines, like Windows' lack of case-sensitivity in its file system).

Bonfire

Practical CodeIgniter 3
CodeIgniter Testing Guide