Swordfish

This is where a number can exist only twice in the same three rows and columns. Consider the following board:

In rows three, six and nine, the only two squares that can contain a 7 are in columns two, five and eight. We can deduce that for columns two, five and eight that a 7 must be in rows three, six and nine. Therefore, we can remove 7 from the possibilities for the rest of the squares in columns two, five and eight.

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