while compiling a level, I got a warning about duplicate planes. The level compiled fine and looked good when I ran it. I was just wondering, if there is no visible error in the geometry, should I worry about this warning?

The block in question was a doorway that was made in TrueSpace and imported into WED. It's an arch.

Its probably one or more double-layered polygons. Look for it VERY carefully,
the texture may be "flickering" or "jitterey" if it is a visible polygon.

If theres only a few its probably OK to leave them, but Im only noobish at directx.
If theres too many, you may start getting randomised directx sorting/zbuffer errors.
And even when there is only a few, it MAY cause performance issues in 3dgs engine.
I would guess if its not important, 'they' would not have the compiler warn about it.

How would I go about fixing this?

open your mesh up and delete the duplicate planes.

Would I have to go into truespace to fix it?

Would it work as a doorway if I brought it in as a model and set its polygon flag on? The player has to be able to go through the arch.