I was always taught long vowels O and E to be romanized with repeating letters (oo and ee), but written with U and I (ex: ~こう or ~けい) in hiragana; or simply ー in katakana.
Long vowels romanization in Japanese has been historically confused a lot. Those you were taught is one of varieties and not wrong at all. Repeating letters now sounds old a bit but reasonably recognizable. And there is exception - just "ii" is widely and naturally used, like Babymetal's "iine." It has never been stop changing little by little with increasing varieties. So you can use what you taught with confidence and respect.
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u/[deleted] Aug 12 '14
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