Excel Referencing

Hi Experts, I am having the following columns in the excel

AA BB CC
REQ_3.1_1 REQ_4.1.1_2
REQ_4.1.1_3
REQ_4.1.1_4
REQ_4.1.1_3
REQ_4.1.1_4
REQ_3.1_2
REQ_4.1_1
REQ_4.1_2
REQ_4.1_3 REQ_4.1.1_5
REQ_4.1.1_6
REQ_4.1.1_1
REQ_4.1.1_2 REQ_3.1_1
REQ_4.1.1_3 REQ_3.1_1
REQ_4.1.1_4 REQ_3.1_1
REQ_4.1.1_5 REQ_4.1_3
REQ_4.1.1_6 REQ_4.1_3


Here. in column BB has values from column AA. It is required that in column CC it should display all the Values from column AA, that has the reference that point in column BB.

Please help me, how can I use excel intelligence to do the same for a list that has more than 1000 points.

Thanks
Sheba Singh