The removal of duplicate faces that are created during the handling of degenerate cases was implemented already but didn't work.
This patch should fix some crashes with the solidify complex mode related to that.

Simplest case that crashes currently can be reproduced as follows:

1. scale default cube along the z axis with 0
2. add solidify with complex mode
3. if it did only give an error not a crash try changing the thickness

There was also crashes with very small geometry when subdivision surface was applied, that should also be fixed now.

One example of the second case can be reproduced as follows:

1. remove the default cube
2. add a circle with default amount of vertices and fill set to nothing
3. in edit mode scale the circle by 0.001
4. press F to make a face
5. search for poke faces (which was Alt+P in 2.79)
6. add a subdivision surface with 3 or more subdivisions
7. add a solidify with complex mode

In this second case the geometry doesn't really work even with the patch, but at least there is no crash.