The error normally shown if a named <ref> is defined multiple times doesn't appear if the page is missing <references/> tag. Spotted by @IKhitron at T85386.
Works: https://www.mediawiki.org/w/index.php?title=User:IKhitron/draft&oldid=1910206
Doesn't work: https://www.mediawiki.org/w/index.php?title=User:IKhitron/draft&oldid=1910205