Why can't you define an external object twice and never use it?

[Doug Gwyn]

In article <54 at looney.twinsun.com> eggert at twinsun.com (Paul Eggert) writes:
->the real problem is that you have violated the "only one
->initialization" constraint.
-Where is this constraint written down?  It's not in either pANS 3.5.7
-("Initialization") or pANS 3.7 ("External definitions"), where I would
-have expected it.  I fear it was so obvious that it wasn't written
-down anywhere.

It's there somewhere; I remember seeing it a few days ago while
looking for something else.  Maybe it's part of the definition
of an object.