I have data that looks like this:

       A              B
1 Account       Notes
 ---------       ------
2 1000           abc
3                   def
4                   ghi
5 1001           jkl
6                   mno
7 1000           pq
8                   rs
9 ...               ...

Is there any way to add in a third column that concatenates the notes rows
into one row?  I know doing this: =B2 & "
" &B3& "
" &B4 in cell C2
will give me what I want, but I have about 12,000 rows to sort through and I
don't want to do this all manually.  Is there a way to create intermediate
columns with dummy variables to say, if A is null, but A+1 is not null,
concatenate column B for all the rows for a particular account?

Thanks.