A repeated number does not automatically mean you have to throw away your sudoku. It is a sign that a placement or a note needs checking. The useful step is to find the first move you cannot justify before building more deductions on top of it.
1. Checking the row but forgetting the box
A number may fit the row and column but still be ruled out by the box. Before writing it in, check all three areas. It is not enough that the number does not repeat: to call it certain, you also need to explain why no other number can go there.
Example: A row is missing 2 and 7. Both fit the column of one cell, but the cell's box already contains a 7. That cell can only be 2. Without checking the box, you would think there were two options.
2. Picking one of two candidates because it “feels right”
If you have narrowed a cell down to “2 or 7,” you have made progress: you know what the rules allow. You do not have to solve that cell right away. Keep both candidates small and look for information elsewhere in the grid.
Writing 2 because it feels right can make later moves look justified even though they depend on that first guess. Keeping possibilities separate from answers helps you see what you actually know.
3. Treating an incomplete note as a final answer
Suppose you are tracking where a 6 can go and note it in one cell. When you return later, that single small number does not make it a single candidate: perhaps 3 and 8 were also possible, but you never wrote them down.
Fix: recalculate every possibility in the cell before turning a note into an answer. Our step-by-step candidate example shows how to check all three constraints.
4. Erasing candidates outside the affected areas
When you place a 5, you can cross out other small 5s in its row, column, and box. Do not erase 5 from unrelated cells across the grid. Removing too much can leave a cell with no candidates even when the original puzzle is valid.
Update your notes with three short passes: row, column, box. Then look for a deduction again. This makes it easier not to mix up the numbers you are checking.
5. Confusing “only number” with “only place”
These are two different reasons to fill in an answer:
- Only number for a cell: after checking its row, column, and box, only 4 can fit there.
- Only place for a number: in a row, 4 can go in only one cell, even though that cell has other candidates too.
To prove the second case, check every empty cell in that row. Seeing a 4 noted in only one place is not enough if your other candidate lists are incomplete.
6. Continuing after a contradiction
If the same number appears twice in an area, do not erase one at random. One may be a printed clue that should never be changed. Identify which numbers you added and check the reasons behind those moves.
- Stop adding numbers.
- Review your latest answers, starting with any move you were unsure about.
- Correct the first mistake you can demonstrate, along with any moves that depended on it.
- Recalculate candidates in the affected areas before continuing.
If you did not keep track of move order and cannot locate the error, returning to the starting grid may be clearer than making blind corrections. A hint about the next deduction can also help without showing the whole solution.