Combining multiple cells with text

Column A are various names of which many are duplicated.
Column B are various text phrases associated with the names in column A.

I would like to make a function in column C that will combine the text in column B into the the corresponding cell in column C that represents the first instance of a name in column A.

So here is an example of what I would like to do:

Column A
Name A
Name A
Name B
Name C

Column B
Yellow
Purple
Red
Blue

Column C
Yellow, Purple
(Blank)
Red
Blue

Any suggestions?