multiple hits for one identifier

We all love the power of vlookup (and hlookup for the initiated), but as we know, when you use them, they take "the first hit" even if there may be many instances of the value in the search range.

My problem is that I've got to look up an ID1# and then
list all the ID2#s that are associated with the ID1.

So, in sheet1 column a, I've got ID1 as #1155. On sheet 2, I've got ID1 as #1155 3 times, and #aa, #ab and #ac associated with them. This list has 4,000 rows, so the thought of manually going through sheet2 and copying over the multiple hits is madness!

thanks for any help you can provide...please email me to let me know you responded on the message board, I'd appreciate that.