Array searching

I never know exactly what to put into the title bar since it is supposed to be descriptive!

I am looking for a formula/function that will return the following:

Say I have an array of data wherein there may be either multiple or unique occurences of values. I want Excel to return a list from that array listing each value only once regardless of how many times it occurs in the array.

For example, the data array contains:
......... A .......... B ............ C ............ D
1 ....(ABC) .... (ABC).... (DEF) .... (GHI)
2 .... (GHI) .... (GHI) ...... (MNO) .... (PQR)
3 .... (STU) .... (XYZ) .... (XYZ) .... (XYZ)

ABCD and 123 are supposed to represent coulmns & rows, they don't quite line up right.

I want Excel to generate a list:

ABC
DEF
GHI
MNO
PQR
STU
XYZ

Just one return for each value.

The number of possible values (ABC, etc.) is around 100, making a formula for each individual value time-consuming. However, the data set is contained within a drop-down list (data validation). In other words, on another sheet is all of the possible values that might be in the desired array. Is this possible?