Riddle: What appears once in every minute, twice in every…

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i-start-with-a-circle-a-triangle-i-become-then-i-am

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i-begin-with-a-pair-yet-add-one-more-then-suddenly-im

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i-have-keys-but-open-no-locks-i-have-spaces-but-no-room

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i-start-with-a-bang-im-never-the-same-predict-my-next

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im-seen-once-in-a-twinkle-and-twice-in-a-pattern-but