This error came up when I tried to use the Mesh Reduce Tool. It would be nice to have either the Project Doctor or the Object Doctor be able to fix these types of messed up (imported) objects or a dedicated 1-2 Sided Tool as was in V6. Currently there is not a tool for repairing these types of objects. I do know a manual method for fixing these without the 1-2 Sided Tool, but when there are many bad objects it becomes a tedious operation.
The manual method is to derive all the faces from the object, restitch them back together, separate volumes and then discard the duplicate faces (those that were the second side of 2 sided faces). The problem with this is when there are objects which have coincident points, segments and/or faces that each object needs to be isolated first and then processed one by one, ugh! A 1-2 Sided Tool or Doctor Tool would handle many objects in a single selection, single operation saving a lot of time, (and a trip back to 2007's FZ6 )