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This question already has an answer here:

I need some examples of sudokus having more than one solution.
Do these puzzles have any common things?

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marked as duplicate by Glorfindel, QuantumTwinkie, Chowzen, gnovice, Sensoray Jul 3 '18 at 15:50

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  • $\begingroup$ I'm not really sure why this was marked as a dupe... to me this question is clearly asking a different thing -- a specific example. $\endgroup$ – timidpueo Oct 24 '18 at 3:29
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The simplest way to make a sudoku with multiple solutions is to find four corners of a rectangle that must have one of two values, as in the example below:

enter image description here

The four cells must be filled with 2 and 7, but they can be filled in two ways.

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    $\begingroup$ Nice answer, but there's a typo: there are two 3's in the bottom middle block. The top 3 in that block should be a 2. $\endgroup$ – timidpueo Oct 24 '18 at 3:40
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According to this study at MIT, the minimum number of cells that need to be populated to generate a solvable Sudoku is 17. If we ignore the definition of Sudoku (which states that there must be precisely one solution), then any valid 9x9 grid with 16 or fewer numbers on it will have multiple solutions.

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