Singleton subtag appearing twice is considered valid

[bitGatito]

This package does not treat the tag en-a-bbb-a-ccc as invalid even though the singleton subtag a appears twice.

According to the BCP 47 specifications: Each singleton subtag MUST appear at most one time in each tag (other than as a private use subtag). That is, singleton subtags MUST NOT be repeated. For example, the tag "en-a-bbb-a-ccc" is invalid because the subtag 'a' appears twice. Note that the tag "en-a-bbb-x-a-ccc" is valid because the second appearance of the singleton 'a' is in a private use sequence.

Is this an issue that needs to be fixed?

[wooorm]

Whether it needs to be fixed depends on how you want to fix it.

Probably depends on how others do it.

But a warning is probably a good idea at least