Minimum reproducible example:

{{< tabpane >}}
{{< tab lang="en" >}}
foo
{{< /tab >}}
{{< tab lang="en" >}}
bar
{{< /tab >}}
{{< /tabpane >}}

This works flawlessly with latest release version v0.7.1. A warning is emitted during run:

WARN  Shortcode "tabpane": duplicate language "en" detected, disabling persistance of language to avoid multiple tab display. Position: ....

With HEAD of repo, tabs are frozen, you cannot switch between the two tabs. There is no warning about duplicate persistence keys emitted any more.

@chalin: you worked on the tabpane shortcode lately. Can you have a look what's going wrong here? Thanks.