I am trying to do the following. Assume data starting in row A1:

AB
AB-C
DEF
DEF
DEF-GH
DEF-GHI
DEF-GHI
JKL
JKL-M
JKL-MN
JKL-MN
JKL-MN

Basically, what I would like to happen starting in B2 (since B1 will
alwaysequal to A1), is that it looks at B2, and if B2 is not a duplicate of
anything above it puts B2, otherwise, if it is a duplicate it moves on and
puts B3, so long as B3 is not a duplicate of anything from above, and so and
so forth. So the answer to the above data would look like:

AB
AB-C
DEF
DEF-GH
DEF-GHI
JKL
JKL-M
JKL-MN