Highlight duplicates based on another column

[wikus]

I would like to highlight duplicates in column B based on column A.
I tried a conditional formula but it's not working.
Any help will be appreciated.

 

[shift-del]

Hi

Mappe3
	A	B
1	a	1
2	a	2
3	a	3
4	a	3
5	b	1
6	b	2
7	b	2
8	b	3
9	c	1
10	c	1
11	c	2
12	c	3
Tabelle1

Cells with Conditional Formatting
Cell	Condition		Cell Format	Stop If True
B1:B12	Expression	=ZÄHLENWENNS($A$1:$A$12;$A1;$B$1:$B$12;$B1)>1	text	NO

Cond. format:

=COUNTIFS($A$1:$A$12,$A1,$B$1:$B$12,$B1)>1

 

[wikus]

Hi

Mappe3
	A	B
1	a	1
2	a	2
3	a	3
4	a	3
5	b	1
6	b	2
7	b	2
8	b	3
9	c	1
10	c	1
11	c	2
12	c	3
Tabelle1

Cells with Conditional Formatting
Cell	Condition		Cell Format	Stop If True
B1:B12	Expression	=ZÄHLENWENNS($A$1:$A$12;$A1;$B$1:$B$12;$B1)>1	text	NO

Cond. format:

=COUNTIFS($A$1:$A$12,$A1,$B$1:$B$12,$B1)>1

Thank you very much for the reply, I appreciate your help.