Question: How do I filter duplicates with a condition?

Answer:

Column B contains category and column C contains Items. Only duplicate Items with adjacent Category number 2 is listed in column E.

AA is in category 2 (row 3) and 1 but exists only once in category 2. It is not a duplicate.

BB is in category 2 and exists twice (row 4 and 9). BB has a duplicate.

CC is in category 2 and has a duplicate (row 6 and 8). CC is a duplicate.

Formula in cell E4:

If you want duplicates sorted from A to Z, read this article:

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Explaining formula in cell E4

Step 1 - Identify values that has not been displayed before

COUNTIF($E$3:E3, $C$3:$C$9)=0

becomes

COUNTIF("Duplicate items",{"AA";"BB";"AA";"CC";"AA";"CC";"BB"})=0

and returns

{TRUE;TRUE;TRUE;TRUE;TRUE;TRUE;TRUE}

In cell E4 no values has been shown before so the array returns TRUE for all values.

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Counts the number of cells that meet a specific condition.

Step 2 - Find duplicates in category 2

COUNTIFS($B$3:$B$9, $F$2, $C$3:$C$9, $C$3:$C$9)>1

becomes

COUNTIFS({2;2; 1;2;1; 2;2}, 2, {"AA";"BB";"AA";"CC";"AA";"CC";"BB"}, {"AA";"BB";"AA";"CC";"AA";"CC";"BB"})>1

becomes

{1;2;1;2;1;2;2}>1

and returns {FALSE;TRUE; FALSE;TRUE;FALSE; TRUE;TRUE}

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Step 3 - Multiply arrays

(COUNTIF($E$3:E3, $C$3:$C$9)=0)*(COUNTIFS($B$3:$B$9, $F$2, $C$3:$C$9, $C$3:$C$9)>1)

becomes

{TRUE;TRUE;TRUE;TRUE;TRUE;TRUE;TRUE} * {FALSE;TRUE; FALSE;TRUE;FALSE; TRUE;TRUE}

and returns {0;1; 0;1;0; 1;1}

Step 4 - Divide 1 with array

1/(((COUNTIF($E$3:E3, $C$3:$C$9)=0)*(COUNTIFS($B$3:$B$9, $F$2, $C$3:$C$9, $C$3:$C$9)>1)))

becomes

1/{0;1; 0;1;0; 1;1}

and returns {#DIV/0!; 1;#DIV/0!; 1;#DIV/0!; 1;1}

Step 5 - Find last matching value in array

The LOOKUP function ignores errors but requires the second argument to be sorted ascending. However our list contains only errors or 1 so the LOOKUP function returns the last matching 1 in the array.

LOOKUP(2, 1/(((COUNTIF($E$3:E3, $C$3:$C$9)=0)*(COUNTIFS($B$3:$B$9, $F$2, $C$3:$C$9, $C$3:$C$9)>1))), $C$3:$C$9)

becomes

LOOKUP(2, {#DIV/0!; 1;#DIV/0!; 1;#DIV/0!; 1;1}, $C$3:$C$9)

becomes

LOOKUP(2, {#DIV/0!; 1;#DIV/0!; 1;#DIV/0!; 1;1}, {"AA";"BB";"AA";"CC";"AA";"CC";"BB"})

and returns "BB" in cell E4. BB is the corresponding value to the last 1 in the array, bolded above.

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Finds a value in a sorted cell range and returns a value on the same row.

Step 6 - Relative cell refences

The following cell reference is both an absolute and relative cell reference: $E$3:E3. When you copy the formula to cells below, the cell ref changes. That way the formula knows which values have been displayed before.

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Get excel example file

Filter duplicates with a condition.xlsx

(Excel Workbook *.xlsx)