In the GSD workbench you can use the 'delete useless elements' command which under the tools menu. This will allow you to remove everything you don't need in one go. It also highlights all the useless elements in your tree and then you can delete them individually. Useless elements are defined as anything that does not go towards making a solid. Don't use it on a pure surface model without taking care
There could be another option, but if all of your geometry is duplicated, it could be more of a headache...
This scenario assumes you are searching for duplicate surfaces. Start making a "Join", give it a coarser tolerance than the default micron, and pick one face only. Then, for a better visualisation, create a boundary of the "Join" feature and maybe give it a different color. Then edit (double-click) the Join feature, and in the "Element to join" window, right-click and choose "Distance propagation". Whenever you have two identical faces, chances are that you will be given the "Fork" message, meaning that the system doesn't know which way to propagate. The missing portions (shown by the boundary and/or the different colors) you have to add manually...
Whenever you have the joined surfaces with only one outer boundary (or with no boundaries if you close a volume), you can erase all the other surfaces that don't make up the Join feature, they are duplicate geometry.
