Noticed the diadem tag when ark_royal_(kantai_colletion) was introduced.
But I also noticed that the same character also receives the tiara very often.
Reading through both wikis, there is not much difference, if at all.
On top of that, there are a lot identical looking "hairbands", but tagged with a different tag:
post #2781886 is tagged with diadem
post #2824396 is tagged with tiara
And those are only some examples. So it seems to me that the usage isn't quite clear of those tags and having both seems to cause some tag noise. There is also not really a difference between the two tags, despite the wiki saying that a diadem has more jewelry. I'd still say, because of the unclear usage of the tags, that one tag should be aliased to the other one.
And tiara has a larger tagcount, so I went that way.
EDIT: The tag alias diadem -> tiara has been rejected by @NWF_Renim.
Updated by DanbooruBot