Localization calls provider twice when fallback matches requested language

Description

Steps to reproduce

1. A resource does not exist in langugage "en", and the site is configured to use fallback mode in localization providers.
2. The fallback culture is "en".
3. The visitor is browsing site in "en" culture.

For single call of the LocalizationService.GetString("..."), the underlying list of providers is called twice - one for the language visitor is currently using("en"), and a second call for the fallback language - "en" again.

http://world.episerver.com/forum/developer-forum/Feature-requests/Thread-Container/2016/10/fallback-language-in-localization-providers/