# Stuck at understanding how duplicate prevention formula works in Google Sheets

Many blog posts and educational sites tell us to use `=countif(\$A\$2:A2,A2)=1` in the selected range.

However, they don't explain it. And for me it doesn't work and since I can't understand it I can't debug it.

My understanding about Sheets is that `\$` is for absolute reference. Thus `\$A\$2` means an absolute reference to the `A2` cell. For me the formula becomes `A2:A2` which is weird.

Can anyone please explain how this formula prevents the duplicates and what is the logic behind it? Thank you.

The range reference `\$A\$2:A2` is using a mixed address mode where `\$A\$2` is absolute and `A2` is relative. When the formula is "dragged down" by copy-pasting cells or applying it to a range in a conditional formatting custom formula rule, the first part stays put and the second part adjusts, like this:
`=countif(\$A\$2:A2, A2) = 1`
`=countif(\$A\$2:A3, A3) = 1`
`=countif(\$A\$2:A4, A4) = 1`
`countif()` will thus get the number of values that match the value in the current row, looking at the range that starts in `A2` and ends in the current row of column A. If that count is `1`, the formula gets `true`. If there are more than one copy of the value in the rows above and including the current row, the formula gets `false`.