I don't think this is an AdSense issue so much as it is a Google Search issue. If you ran afoul of Google Search's duplicate-content filter, you could lose traffic and revenue.

I have a .net and a .com with the same domain name that point to the same site. Would that also be a problem?

absolutly. Try asking this same question up in the Google search forum.

If I put the same content on both domains what would be the effect with AdSense?

This really isn't an AdSense question because AdSense doesn't care.

Google however, if it notices both domains contain duplicate content will probably make one of them supplemental so in essence you'll only be left with one web site fully functional driving traffic.

I have a .net and a .com with the same domain name that point to the same site. Would that also be a problem?

No.

As long as they fundamentally function as a single website with the .net redirecting to the .com, not a problem.

Dunno about AdSense but I can "absolutely guarantee" Yahoo will have a HUGE hate upon you.

Not that I've ever cared. But, you know what happens.

[Added] I can't really blame Yahoo, just that "dopey" Inktomi.

I await the flames...

I've paid the consequences of my own actions and I don't ever do any "black hat" stuff. Just evolutionary of 11 years.

I steadfastly go down my own road. Most people in my experience won't do that.