Pages in [[Category:Pages using duplicate arguments in template calls]] (see https://www.wikidata.org/wiki/Q18338361 ) should show which template and which parameter are the duplicates. Pages are added to that category automatically through some undocumented process, and at least on Commons, for a lot of pages it is close to impossible to trace which template and which parameter are the duplicates. So I would propose to show some error code in red with that information.
Related Gerrit Patches:
|mediawiki/core : master||Warn when duplicate arguments are found|
It has been 3 months. Any chance this patch can be rolled out. There are 74k pages in https://commons.wikimedia.org/wiki/Category:Pages_using_duplicate_arguments_in_template_calls, and some duplicated are very hard to find.