"Not once but twice" triggers subj-aux inversion?

[Neal Whitman]

>From Charles Krauthammer's column yesterday:

"Not once but twice (Afghanistan and then Iraq) did Bush seek and receive
congressional authorization, as his father did for the Gulf War."

Why the subject-auxiliary inversion? With "Not once" I get it, but not with
"Not once but twice," which isn't a negation of the main verb.

Further discussion and additional attestations on the blog:
http://literalminded.wordpress.com/2011/06/27/not-once-but-twice/

Neal Whitman
Email: nwhitman at ameritech.net
Blog: http://literalminded.wordpress.com

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