Make sure Language::getTranslation removes duplicate keys

- [ ] Instead of calling `CM_Model_LanguageKey::create` introduce a new `CM_Model_LanguageKey::createAndRemoveDuplicates` (which would have code from `CM_Model_LanguageKey::replace`)?
- [ ] CM_Paging_Translation_Language: `$groupBy = 'BINARY k.name';`
- [ ] Also run a one-time script to remove duplicates.
- [ ] Also: `LanguageKey::_getContainingCacheables` not necessary?

cc @alexispeter 
What do you think about wrapping this into method?

Method name could be `findSingle($name)`, `findNonDuplicate($name)`, `findByNameNonDuplicate($name)`.