Displaying A Message Box

I have found several examples for checking whether the value entered into a cell is equal to that in another cell and if not displaying a Message Box.

What I am struggling with is how to extend this to a range (array) of cells.

For example, if the value entered into cell A1 equals data already in cells C1, D1 and E1 then a Message should be displayed saying something like ‘number already used.’

The code should also be capable of performing the same check when entering data in cells A2, A3, etc.

Please can anybody help me.

Thank you.

Thank you for this.

I have a further question, can the code be expanded so that data entered in columns A and B for example are checked against data in C, D and E.

Thank you.