Cells you do not shade stay clear. Shade extra copies until each remaining number appears only once in its row and once in its column. Shaded cells cannot share an edge, and all clear cells must stay joined by sides.
When three matching numbers sit in a line and share edges, the middle one must stay clear and the two outer copies must be shaded.
Before: the fifth column contains three 4s in a row. At least two copies must be shaded.
After: the outer 4s are shaded and the middle 4 stays clear. If the middle 4 were shaded, both outer 4s would stay clear and repeat.
Build chains from clear cells
Use the sandwich patternWhen matching numbers have one cell between them, the middle cell must stay clear. Shading it would force both matching cells to stay clear, so the same number would appear twice in that row or column.Mark every shaded neighborA shaded cell makes the cells above, below, left, and right clear. Add those notes at once.Let one clear copy settle the restWhen a copy is confirmed clear, every matching number in the same row or column must be shaded.Protect narrow connectionsIf shading a cell would isolate a confirmed clear cell or split clear areas with no other path, that cell must stay clear.
A duplicate gives a choice, not an answer
Two matching numbers tell you that at least one must be shaded. They do not tell you which one. Check the row and column of each copy, its shaded neighbors, and the clear-cell path before you choose.