Steps to replicate the issue:
Edit any Wikisource/Commons page and enter one of the following lines. Then save the page. If you use the old Vector skin, previewing your changes is sufficient.

If you disable Compact Language Links in "Special:Preferences → Appearance" you can observe this bug in more Wikimedia wikis (not just Wikisource and Commons).

[[gsw:]] [[als:]]
[[yue:]] [[zh-yue:]]
[[lzh:]] [[zh-classical:]]
[[nan:]] [[zh-min-nan:]]
[[nb:]] [[no:]]
[[mo:]] [[ro:]]
[[rup:]] [[roa-rup:]]
[[sgs:]] [[bat-smg:]]
[[vro:]] [[fiu-vro:]]

(This list is probably incomplete)

What happens?:

Two interlanguage/interwiki links are added. This bug can currently be observed on https://commons.wikimedia.org/wiki/User:HTinC23/Sandbox

What should have happened instead?:

One interlanguage/interwiki link is added because e.g. "gsw" and "als" are the same language and the same wiki.