Code to search for duplicate and pop up message asking for different value

Hi everyone,

I need a piece of code that can be added to a macro I have written for a "save button" which when pressed checks the contents of cell C15 on sheet 1 of my workbook against all values in column A on sheet 7 for a duplicate and if it finds one shows a pop-up box asking the user to enter a different value to avoid having multiple items with the same reference number.

Thanks in advance,

Glen