Select column A data validity-Settings-Allow-Customize-Formula: Enter.
=countif(A:A，A 1)& lt； 2
Error Warning Select Stop or Warning.
Set the required warning information.
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Not when copying. You said input, not plagiarism.
Try double-clicking the copied cell.
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In addition, I'll tell you a statistical repetition formula.
Suppose the data is in column a.
B 1 input
=countif(A$ 1:A 1，A 1)
Drop-down formula
Select column b filter-automatic filter-drop-down box. If it is greater than 1, it is a duplicate.