Each section of a page gets a unique HTML id in their header based on the section header text. If there are multiple sections with the same header text, the second ID gets a _2 suffix, the third a _3 suffix, etc. That's good, but it causes a conflicting ID when there's another section with the same header text and a " 2" suffix, etc.
The problem, apart from the duplicate HTML id, is that the TOC is unable to link to one of those sections.
This problem is common on english wiktionary, because they have duplicate section names very often: They use "Etymology" for each language, and if one language have several homonym terms, one is named "Etymology", the second "Etymology 2", etc.
Galician->Etymology has the same ID than Latin->Etymology_2
Catalan->Etymology_2 has the same ID than Dalmatian->Etymology