(EDIT: I've decided to move this from the puzzles by AO thread so that people would not think that these have the same authorship. I don't know who composed this puzzle but like many mate in > 3 puzzles it is deliciously contrived. I've titled the thread "various problems" as I hope to post more puzzles of this type at the rate one per week or so.)

Have we seen this one before?

White to move and mate in 6.

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