If a character plays multiple roles in a movie (ie Coming to America) is it possible that they can get nominated more than once for an oscar for the same movie? Maybe one role was a lead and the other a supporting role. I remember Julia Roberts being nominated twice but that was for two different movies.

Just to be on the safe side, I don't think Eddie Murphy should have been nominated for Coming to America.

Seems like they might:

6. In the event that an actor or actress receives a sufficient number of votes to be nominated for one achievement in one category and for another achievement in the other category, both achievements shall be eligible.

from http://www.oscars.org/75academyawards/rules/rule06.html

Hmm. It seems that Nicholas Cage was nominated for BOTH roles he played in Adaptation.

Hmm ... here's another rule from the Academy, which may more directly address the original post:

5. In the event that two achievements by an actor or actress receive sufficient votes to be nominated in the same category, only one shall be nominated using the preferential tabulation process and such other allied procedures as may be necessary to achieve that result.

So, for example, Jeremy Irons could not have received two nominations as Best Actor for playing each of the brothers in Dead Ringers, and Julie Christie could not have been nominated Best Actress in Fahrenheit 451 for both Clarissa and Montag's wife.

5 and 6 kind of contradict each other. I think 5 is saying that if Jeremy Irons had a lot of votes for Best Actor in both The House of the Spirits and M Butterfly, he would only receive the nomination for one of those roles. 6 seems to say that if he received a lot of votes for Best Actor as Elliot and Best Supporting Actor as Bev (both in Dead Ringers), that he could receive both nominations.

I don't think they're contradictory at all - the nominations are by actor, not by role. As mentioned already, Nicholas Cage got nominated for Adaptation, but no one said for what role - his nomination came based on his performance in the film in total.

Another example is when Peter Sellers got nominated for Dr. Strangelove. (He should have won.)

Even though a Director/Producer/Screenwriter can be nominated in all 3 categories. I don't think Nicolas Cage could have been nominated for Lead Actor as Charlie Kaufman and also for Supporting Actor as Donald Kaufman.

5 and 6 are definitely speaking of Numanoid's comments.

not about the eddie murphy thing.

and we all understand tha eddie murphy will probably never be nominated for an Oscar.

you are most definitely safe

Which is why certain positions are campaigned for with certain actors. For example, Julianne Moore was nominated for supporting actress for The Hours and best actress for Far From Heaven to give her better chances.

Worse case was Nicole Kidman who was nominated for Moulin Rouge last year as best actress when she'd have had a better shot with The Others but Miramax agreed to bow out of their campaign.

This assures that the actor doesn't split their own vote and keep themselves out of the category completely. I believe the agent negotiates this.