As @b2m says it looks like the “Blank down” process in step 5 hasn’t worked as expected. Once you’ve done the blank down I’d expect your project to look something like:
Note that I was in “Rows” mode when I did the “Blank down” operation, and now I’ve switched to Records mode - which you can see has grouped together the rows that had the identical DOIs - and those duplicate DOIs no longer display.
Running the “Join multi-valued cells” on the Source column at this point works as exepected for me
Please do share some screenshots from data in OpenRefine at different stages of the process if you are still having issues as it may help show what’s going wrong