 If you liked
xxxHOLiC Movie: Manatsu no Yoru no Yume
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 ...then you might like
Tsubasa Chronicle: Tori Kago no Kuni no Himegimi
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Both are created by CLAMP, plus there's a similar/connected scene that can be seen on both movies. However, I recommend seeing Tsubasa Chronicle movie first as xxxHolic movie carries somewhat of a spoiler for the former than the latter.
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Recommended by kunoichi_hime
They are both by Clamp, and you'll know where the key that Yuko receives goes if you watch the Tsubasa movie..
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Recommended by Chicken008
They're both CLAMP productions that came out on the same day. There's some mystery or twist in both of them.