Ok, i can’t be sure what caused it but here’s what i’ve found so far.
the error occurs in safe mode with all plugins, and also with the theme turned off. that doesn’t mean its a bug in stock/our install of stock as its probably more likely one of the plugins put the duplicate tag(s) into the database and they are still there causing problems when the plugin is turned off.
to correct it i’ve been searching for duplicate tags and changing them to something else. (‘web-dev’ into ‘webdev’)
i’ve noticed that some tags are set as posts on the tags, but aren’t listed. so the site will tell me i have 12 remaining web-dev tags, but the search function can’t find them.
many posts have both identical tags in the tag entry box, and deleting one removes the other.
my best guess so far is that the tags causing problems are also put as links in the post itself, so when someone puts #web-dev in the text body of the post it is creating a tag, and if that tag is already in the title of the post it is creating a duplicate.
I’m going to try and force that to occur and so i can observe it.
Something else is going on though as if you look at this search result, the top post does not contain ‘web-dev’ as i’ve already changed it, yet the search function is still seeing it there somewhere.
Edit: ignore the last part, the search was just being intuitive. the same search for this " web-dev " removes the false positives.
..but ‘suggested topics’ is still finding tags which ‘search’ can’t see:
