VBA to check new entry for duplicates and then copy present values

Hi all.

I know that similar problems can be found around the web, but I am new to the whole VBA thing and thus thought it better to ask.

I have a range in a worksheet, A2:A10000, which contains reference numbers (let's called them X). What I need is that when a reference X gets entered into any cell of that range (A2:A10000), a macro will check if that reference X is anywhere else in that range, aka look for duplicates. IF there is a duplicate, I need an option popup box that asks to copy values from that duplicate (Yes or No). If selected Yes, I would like values from columns B and C, but the same row as the duplicate, to be copied to the new entry made.

As an example, let's say that a duplicate is entered in A30 that is also found in A27. What I then need is to copy the value from B27 to B30 and the value from C27 to C30.
I'm fairly sure this can be simply coded, but I haven't found a way to work around the row-column indexing nor the popup box.