Thanks for the help.

I understood the idea. I have implement it in my sheet where I combine
it with two lists, depending on when I choose in the combo box left of
it:

List 1: AA BB CC DD EE FF
List 2: 10 11 12 13 .. 20

Value X = via combo box = 1 or 2

X=1 choose list 1
X=2 choose list 2

So far I actually add another dimension. It works, just have two
checkname data definitions (see
http://www.contextures.com/xlDataVal03.html)

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But I need to go one step further:

Not only check wether the value is choosen in the present worksheet,
but also if it is already choosen in a similar location but on another
worksheet.

Data choose at Worksheet 1

X=1 AA
X=2 12
X=1 BB
X=2 17

Data choose at Worksheet 1

X=1 DD
X=2 10
X=1 I only can choose CC or FF
etc.

Anybody can assist me, I couldnt find out.

Bart