Greetings:  I am married, and my wife and I live and work in
California.  Each of us has health insurance coverage through our
respective employers, and we are each listed as covered dependants on
each other's coverage.  Thus, we each have "double coverage."

Every six to eight months I get what seems to be a routine letter from
one of our providers or the other, asking us to identify any other
health coverage we might have through spousal coverage, etc.  These
letters cite no reasons for the question, nor do they cite any law
allowing them to ask, nor do they assert any legal authority to ask. 
They simply say, in effect, "We are updating our records.  If you have
additional health insurance coverage through a spouse or other policy,
please provide the details of that coverage below and return this
letter to us in the enclosed envelope."

Today, my wife's employer (a large school district) sent a form to all
of their employees, asking them to "confirm" that their coverage
information (which was provided on the letter) was correct, and notify
them of any changes.  Then, to my surprise, at the bottom of the form,
was the same question I mention above.

Perhaps I'm paranoid, but I find this a bit disturbing.  I hear rumors
about insurance companies refusing to pay, and/or employers refusing
to provide or cancelling coverage because of the presence or
availability of other health coverage.  I wonder whether or not the
law requires disclosure, or whether privacy laws make it illegal for
them to ask.

I've spent all evening checking Google for any relevant information,
and must now hand my question over to the experts.  With this
background in mind, my question is:

Am I required to disclose the existence of other health care coverage
to any of my health insurance providers or employers, would the
providing of such information be a good or bad thing, and why?

I hope I've provided enough detail - this is my first Google Question
- and, to me, it's pretty important.  At a minimum, I need to know
what the laws say, if anything, and I'd be especially appreciative for
any analyses of the pros and cons of this situation.  Thank you for
your time and effort on my behalf.

When a subscriber (in this case your wife) enrolls into a healthplan,
the fine print specifically states the subscriber will disclose other
coverage. If, at some point down the line, said healthplan pays claims
for a dependent and then determines there was other coverage on the
dependent, the healthplan will request a refund. Each state has
different rules about the time lapse, where they can no longer request
refunds from providers. Essentially you and your wife agreed to
disclose this info when you enrolled. Hope this helps.