The image above demonstrates a conditional formatting formula that highlights duplicate items based on date. The first instance is not highlighted, only subsequent duplicates are highlighted.

Conditional formatting formula:

Explaining Conditional formatting formula

Step 1 - Concatenate cell values

The cell references $C3 and $D3 are locked to each column, however, they change when CF move to the next row. The ampersand sign concatenates the values in these cells.

$C3&"-"&$D3

becomes

40184&"-"&1150

and returns

"40184-1150"

Step 2 - Concatenate values using expanding cell references

These cell references expand as the CF moves to cells below, the formula keeps track of previous values so it can detect duplicate values.

$C3:$C$3&"-"&$D3:$D$3

becomes

40184&"-"&1150

and returns

"40184-1150".

Step 3 - Compare concatenated values

The equal sign lets you compare values, the result is always TRUE or FALSE.

$C3&"-"&$D3=$C3:$C$3&"-"&$D3:$D$3

becomes

"40184-1150" = "40184-1150"

and returns TRUE.

Step 4 - Convert boolean values

The SUMPRODUCT function can't work with boolean values, we must convert it (them) to their numerical equivalents. TRUE = 1 and FALSE equals 0 (zero).

--($C3&"-"&$D3=$C3:$C$3&"-"&$D3:$D$3)

becomes

--(TRUE)

and returns 1.

Step 5 - Sum values

The SUMPRODUCT function sums the values in the array in this step.

SUMPRODUCT(--($C3&"-"&$D3=$C3:$C$3&"-"&$D3:$D$3))

becomes

SUMPRODUCT(1)

and returns 1.

Step 6 - Check if duplicate

This steps checks if the sum is larger than 1, if it is then TRUE is returned and the cell is highlighted.

SUMPRODUCT(--($C3&"-"&$D3=$C3:$C$3&"-"&$D3:$D$3))>1

becomes

1>1

and returns FALSE. Cell C3 is not highlighted.

Get Excel *.xlsx file

Highlight-duplicates-within-same-date-week-month-year.xlsx

Highlight duplicates on same week

Conditional formatting formula:

Highlight duplicates on same month

Conditional formatting formula:

Get excel sample file for this article.

Highlight-duplicates-within-same-date-week-month-year.xls
(Excel 97-2003 Workbook *.xls)

Functions in this article:

SUMPRODUCT(array1, array2, )
Returns the sum of the products of the corresponding ranges or arrays

YEAR(serial_number) returns the year of a date, an integer of the range 1900-9999

MONTH(serial_number) returns the month, a number from 1 (January) to 12 (December)

WEEKNUM(serial_number, return_type)
Returns the week number in the year