Raised by @Umherirrender at change 1004305:
At the moment the call to Parser::fetchTemplateAndTitle (directly or via Parser::getTemplateDom) would register the interwiki as template via ParserOutput::addTemplate and it gets stored in the linktarget table with ns=0 and title=Non-Wiki-Part. When this is a interwiki to a user page, the linktarget has ns=0 and title=User:Name and namespaceDupes.php wants to repair this (the way I have seen this - T355706: Using interwiki on {{PAGESIZE:}} stores external title in linktarget).
I'm creating this task mainly for DBA awareness, and to sollicit code review from CTT and/or someone else in MW Eng.
LabeledSectionTransclusion is a parser extension, although it is currently listed as unowned at mw:Maintainers.