I have a list of 500 names in column A.
1 name1
2 name2
3 name3
4 name1
5 name2
6 name3
7 name1
8 name2
9 name3
..
And i need to
Without using VBA - you can't edit the cells themselves, except using formatting.
You could however, use a helper column and use a formula on these lines to accomplish this:
=IF(A2=A1,CHOOSE(RANDBETWEEN(1,26),"A","B","C","D","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z")&" "&A2,A2)
Bear in mind that because this is random, there is a small chance of two consecutive letters - so you may need to use another formula to check for this and make a small amount of manual tweaks.
If you prefer to use a non-random prefix that just contains the next letter, you could do something like this starting in C2:-
=IF(B2=B1,CHAR(96+COUNTIF(B$1:B1,B2))&"."&B1,B2)
I'm assuming that the names are sorted, have a heading and are in column B.
If you didn't want to sort them, this would still work with a modification:-
=IF(COUNTIF(A$1:A1,A2)>0,CHAR(96+COUNTIF(A$1:A1,A2))&"."&A2,A2)
I'm assuming that the unsorted names are in column A, with a header.