@melissalove Sometimes (aka long ago, since they don't anymore) they have the same strategy of releases, but that has changed a long time ago: Ayu doesn't releases that much singles anymore, Namie doesn't takes a lot of time to release albuns anymore, Kumi also doesn't releases more than two or three singles in one year anymore.
It was a normal thing to get a release pattern "4 or 5 albuns / 4 or 5 years = release a compilation", but that also doesn't happen anymore with the three of them, but I can understand what you're saying. It's not like they're copying each other, it's a regular release pattern.
And btw, Kumi's Out Works was a two disc album with remixes (which this idea was dropped before the release), and was a compilation of songs that aren't from her discography, so yeah, it's not the same of BEST FICTION or A COMPLETE.
About the vocal lessons, they probably always had somebody to help with that. Ayu, Kumi and Namie vocals changed a lot during their careers, but they usually aren't really vocal about taking lessons. Kumi is probably trying to get some hype, like she did during the release of Darling. She said the songs would have a really high note (which wasn't that big deal, btw) so she probably did vocal lessons back then.
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The cover os ViVi's latest issue is the model Rola (this name always makes me laugh haha). So yeah, no Kumi on the cover.