Gareth,

Conditional formatting is what you want.

Assuming we are looking at A1:A100, select those cells

Goto FormatConditional Formatting
Change Condition1 to FormulaIs
Add this formula =COUNTIF(A$1:A$100,A1)1
Click the Format button
Select the Pattern Tab
Pick a blue colour
OK
OK

That should do it.

--
HTH

-------

Bob Phillips
... looking out across Poole Harbour to the Purbecks


"Gareth" wrote in message
...
I have customer numbers in column A of a sheet. I would like to be able
to
colour the cell blue for instances when the same customer number occurs
twice.

Thanks in advance.

Spike