promotion with 2 domains

[New]

i heard that when you have one website under two domain names (just the same exact website), that the search engines won't list you anymore.

But i see that different companies have two domain names with one website and still are being listed.

We want to do this, but if se's won't list our website anymore we aren't gonna do this. Then we just modify the website slightly and have two ALMOST the same website under two domain names?
Does anybody knows what you have to change (were the se's will look at and say: this is the same website under two domain names)????

If anybody knows i would be glad to knoww!!!
thanks in advance for the help people!!

[cyanide]

Well, yes search engines will ban duplicate sites.

If some people are doing it.... then it's only a matter of time before the engines find out.

One thing you can do, is set up your site on one domain, then with the other domain(through the registrar) - just do a pointer/forward.