The key displayed in the "Cite error references duplicate key" message has spaces replaced with underscores. This can make it hard to find references with that key on a page with the browser's built-in search. I've made an example on testwiki. Ideally, the reference key displayed in the error message should be exactly the same as the one used in the page source.

This was mentioned on enwiki's Village Pump and on enwiki's citation help talk page.