Make a list of exact match duplicates in large spreadsheet

Hello,
I have a very large list of part numbers (26k+) that I need to load into my system at work. I can't load duplicates into the system which this list contains. I do however need to keep the duplicates and add a prefix.
I've tried the simple conditional formatting of highlighting duplicate cells but I have run into a problem. Many of the part numbers contain the same series of numbers forcing them to highlight whether they are duplicate or not.
For example, many numbers begin with 012 or 022, so the highlighted cells would include:

0120
0123
012
022
0220
0224

I have several numbers that are not duplicates as a result. Thank you for your help.

If all your data starts in A2 use this formula in B2

=countif($A$2:A2,A2) and copy it down. After copying it down, copy and paste vales to remove the formula

When it finds a new number it will return 1, second time 2 etc this can also help with adding a prefix.

I just clicked on conditional formatting, highlight cell rules, highlight duplicate cells

I don't think that worked, I'll give the results I very well could have done something wrong.
I attached the results in its own spreadsheet

This is the entire list of part numbers, sorry I should have attached sooner.

Originally Posted by martindwilson

what prefix do you want to add? it should be possible to do it all in one go with a formula, eg to add a,b,c....to front of those which are duplicates
something like
=IF(COUNTIF(A:A,A1)>1,CHAR(COUNTIF($A$1:A1,A1)+64)&TEXT(A1,"0"),TEXT(A1,"0"))

I'm not sure which prefix I'm going to use, my plan was to add a 2 or 3 letter prefix based on different vendors for the same part number.

I'm sorry, I used the formula:

=COUNTIF($A$2:A2,A2)>1

and what I posted is what was pulled out as "TRUE" values from the Larger list.

After looking at it, I sorted the numbers by value, hoping to see the duplicates next to each other. While there are some right next to each other, there are some by themselves.

What I'm really wanting to do is to somehow group all of the duplicates together within the part number column (column A) so that I can edit them easily.

I attached my original results when I tried this yesterday. I came to this by clicking conditional formatting --> Highlight cell rules --> Duplicate Values. Then I sorted column A by cell color (light red) to be on top, then by column A value. When you look you will see some values do not have a duplicate, I'm confused as to why this is.

I would really like all of the duplicates grouped together in the same column as the other part numbers. kind of like it's shown in the attached file.