[plt-scheme] duplicate structure types

If I define a struct type in a module, then both require and
require-for-syntax that module, then try to access at runtime a struct
that was created at expansion time, I get an error like this:

  foo-bar: expects args of type <struct:foo>; given instance of a
  different <struct:foo>

How can I make it so only one struct type get created?  Or should I
just go back to storing the fields in a cons?

--dougo at place.org