How to show one distinct value instead of multiple results?

Hi,

I'm using a reference module to allow users to attach a certain node to a parent. What I would like to do is show a list of parent nodes that have been 'attached to' by the user. I can do this but my problem is that I can't show just one distinct value if a user has attached more than one node to the parent.

For example if a user attaches 3 nodes (a1, b1 and c1) to a parent node (1), and 1 node (a2) to another parent node (2), what I want to display is:

1
2

...which shows the parent nodes that the user has referenced, but instead I get:

1
1
1
2

...which is what I'm after but I want to group the values that are the same.

Can anyone explain how I go about doing this?

Thanks for any help.