First off, a little bit of nagging. This should have been posted in the Beginners section.

Anyway, the duplicate planes error usually happens when you have two (or more) blocks in the same position/location in your map. Here's how to fix it... sometimes. Build the map again and when it shows the error, note what block numbers it gives. Stop the build. Goto Edit->Find Object in the WED menu. Enter the number and hit okay. The block in question will be highlighted in WED. You may need to scope up/down to get the block. Make sure that you have saved your map and delete the block. Try the build again and see if you still get the error. If so, undo the delete and keep searching.

I usually group all of my "structures" in my map. Each building is its own group, each road, etc. If you do this, you can delete entire sections, build to see if the error is still present and so on.

-Jeff