Help!

Help if you can please!! Here's my problem, I'm trying to identify
multiple values in column B for a unique value in column A.

Column A Column B

100 5687
100 5687
100 5687
100 8569
100 8569
200 4544
300 8563
300 5686
300 1256

So in this example, I'm trying to find all the values in column B that
relate to the unique value of 100 in column A.

Thanks for your help!

Cheers
Chris