Druid wrote:

    How many have people out ther come up this wonderful and unique
story or plot point, and later found out that, while for you it's new,
elsewhere it's been done to death?

    Take for example Ranma his female side get split into two. Now for
me this was a totally new idea at the time I thought of it, but since
that time I've seen messages bashing the entire concept as done to
death. How many other people have had such things happen? I would point
out that the stories might have nothing to do with each other except for
only one thing, but it still is put down for using it...

The major risk in using a frequently appearing plot device, is that
you end up having your work compared to people who are just plain
better than you are.  Apart from that, if enough different variations
of an idea are out there, you may find that no matter what you write
an experienced reader has encountered something remarkably similar.
However, there are always new readers unencumbered by familiarity
with the versions of this idea in Two Sides of The Coin, Tales of 
Shampoo, Nullifier, a couple of other fanfics I've read and written, 
and the two times the idea was used by Rumiko Takahashi, so there's 
always an audience out there for another iteration of a good idea, and
if the execution is sufficiently good, even those who are very familiar
with the idea may enjoy it.  You shouldn't necessarily let the grumbling
of the old lags stop you from doing what you want to do.  

However, if the "only thing in common" is the gimmick around which the
whole plot revolves, the story will likely have a significant amount in
common.  That's just the way it is.