Alex,
it might be a bit confusing, but localization has some nuts and bolds.
As Vincenç stated, Language Packs are outsourced to the community, some members provide them for free (like I myself the German ones at DeutschNetNuke), others sell them at Snow. These language packs contains translated versions of the default resource files, that has to be provided by the core, module and provider developers.
Duplicate keys do currently not lead to a misfunction of the module, but make translators' lifes harder.
For an en-US language pack, you do not need to copy every item, if you edit /App_GlobalResources/Locales.xml, so your English item looks like
<language name="English (United Kingdom)" key="en-GB" fallback="en-US" />