"concatenate" function

[boball]

How can we concatenate column values from multiple rows into a single values.
Just like we have SUM to add column values from multiple rows, is there a way to concatenate the values instead?

More specifically, I want only unique values to come.
For example, if my rows are:
1 A
1 B
1 A
1 A
1 C
2 B
2 C
2 C
3 D

I want the output to be:
1 AB   (not ABAAC)
2 BC    (not BCC)
3 D

i.e. concatenate and show only unique values if there are repeats