In a 3x3 block of a Sudoku puzzle, it may happen that a specific digit cannot be determined its exact position yet, but it is locked in a few cells as a candidate. That means the digit must be in one of a few cells. If these cells happen to fall in the same row, then this digit cannot be a candidate for other cells in that row.
Let's demonstrate this more clearly with an example.
2
4
5
6
8
3
4
5
8
9
3
4
6
8
3
4
9
3
4
6
7
2
5
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8
3
5
8
3
6
7
8
1
3
5
1
3
5
6
2
3
6
3
6
7
2
3
6
7
1
3
5
6
7
1
2
3
6
9
3
4
8
9
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3
6
9
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4
6
9
1
2
3
6
3
8
1
3
6
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5
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3
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8
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3
7
1
3
7
8
3
7
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8
3
7
7
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8
9
In block B4 of this Sudoku puzzle, the digit 3 is a candidate for two yellow cells (R6,C2) and (R6,C3). In other words, the digit 3 in block B4 is locked in these two yellow cells, and one of them must be 3.
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6
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3
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3
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7
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3
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7
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3
5
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7
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6
9
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9
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3
8
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3
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The two yellow cells (R6,C2) and (R6,C3) happen to fall in the same row R6.
2
4
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5
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3
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3
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7
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7
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9
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3
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9
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6
3
8
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3
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9
Since one of the two yellow cells in row R6 must be 3 and each row cannot contain the same digit more than once, other cells (marked in orange) in row R6 cannot be 3. Therefore, the candidate digit 3's in these orange cells can be safely removed.