find values and combine in one cell

[marlea]

Hi-

My data set consists of project titles and their project codes; not all
projects have codes and some have multiple codes:

column A / column B
Force / sci.2
Cells / bio.5
Oxygen / bio.1
.........../ chem.4
.........../ sci.6
.........../phys.7
Trees
Mass / sci.2.5

Problem: In the example above, "Oxygen" has 4 codes; for projects with
multiple codes, how can I combine all the associated codes in one cell
in column C? This is how the example above should look:

column A / column B / column C
Cells / bio.5
Oxygen / bio.1 / bio.1, chem.4, sci.6, phys.7
.........../ chem.4
.........../ sci.6
.........../phys.7
Trees
Mass / sci.2.5

-Thanks for your help