glibcLocales: Fix build for duplicates in locales list
glibcLocales: Fix build for duplicates in locales list

Without this patch, setting the same locale twice, e.g. like this in
NixOS:

```nix
{
  i18n.supportedLocales = [
    (config.i18n.defaultLocale + "/UTF-8")
    (config.i18n.defaultLocale + "/UTF-8")
  ];
}
```

Would make the glibcLocales build fail with

```
Error: unsupported locales detected:
en_US.UTF-8/UTF-8 \
You should choose from the list above the error.
```

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