Started with a box brush, got it to the shape I want, then created a static mesh from it. Then edited static mesh to set “lightmap coordinate index” to 1, added collision, and saved.
Doing an alt-drag to duplicate the original object that has been turned into an SM results in an invisible duplicate, with no static mesh assigned.
A second copy of it, added from the content browser, duplicates properly.
If multiples are selected, none of them duplicate properly if the original is included in the alt-drag.
So the workaround is simple and obvious, but it still seems like something the devs would like to know about.
p.s. Why does setting lightmap coordinate index to 1 need to be done manually? Is there ever a case where one would want it to be 0?