So... is Nanaka 6/17 finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 27, 2003
Manga finished: June 19, 2003
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Nanaka is a bright and focused high school student who often clashes with her more carefree friend, Nenji.
However, an accident drastically changes Nanaka's life, causing her to lose her memories and essentially become a child again.
Now, Nanaka, her father, and Nenji must navigate the challenges of keeping her condition a secret while helping her adjust to her new reality and try to regain her former self, all while experiencing life from a completely new perspective.