This post is taken from a few sources, one of which is Understanding 
Crim, pgs. 261-292; if you have it, READ IT!

********************* D U R E S S (an Excuse) ***********************

It is a defense to a crime OTHER THAN HOMICIDE that the defendant 
reasonably believed that another person would imminently inflict 
death or great bodily harm upon him or a member of his family if he 
did not commit the crime.


ELEMENTS
1) Another person threatened to kill or grieviously injure the actor 
or third party, particularly a near relative, unless she commit the 
offense

2) the actor reasonably believed that the threat was genuine

3) the threat was PRESENT, IMMINENT and IMPENDING at the time of the 
criminal act

4) there was no REASONABLE escape from the threat except through 
compliance with the coercer

5) the actor was not at fault in exposing herself to the threat.



DURESS is NOT a DEFENSE TO HOMICIDE.  
Under CL, duress is not a defense to an intentional killing.  A very 
few states recognize an imperfect duress defense which reduces the 
offense of the coerced actor to manslaughter.



Model Penal Code- 
The MPC defense of Duress is broader than the common law in various 
respects.  First it abandons the CL requirements of deadly force and 
imminency; the defendant is excused as long as the person of 
reasonable firmness would have committed the offense.  Second, the 
defense is one of general applicability, so the defense may be 
raised in murder prosecutions.  Third, the person they are 
threatening does not need to be the defendant or a member of her 
family.


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*************  N E C E S S I T Y (a Justification) ******************
aka "choice of evils" or "lesser evils" defense

It is a defense to a crime that the person REASONABLY BELIEVED that 
commission of the crime was necessary to avoid an imminent and 
greater injury to society than that involved in the crime.  

The test is objective; a good faith belief is not sufficient.


ELEMENTS
1. Actor must be faced with a CLEAR and IMMINENT Danger

2. Defendant must expect as a reasonable person, that his action 
will be effective in avoiding danger; there must be a direct causal 
relationship between his action and the harm to be avoided.  

3. Defendant may not successfully claim necessity if there is an 
effective legal alternative for averting the harm.

4. The harm the defendant will cause by violating the law should be 
less serious than the harm that he seeks to avoid

5.  Legislature must not have precluded the choice


LIMITATIONS:
1.  -Death-- causing the death of another person to protect property 
is never justified.
2.  -Fault-- The defense of necessity is not available if the 
defendant is at fault in creating the situation requiring that he 
choose between the 2 evils.



NECESSITY as a defense to Homicide
Based on Regina v. Dudley and Stephens (casebook p.542- read it!), 
some commentators have concluded that the CL does not recognize the 
defense of necessity in homicide prosecutions, and some statutes 
expressly so provide.

However, it is at least plausible that a court might justify a 
homicide of an innocent person in necessitous circumstances.



***********************  MODEL PENAL CODE-
The MPC defense of necessity is broader than the CL.  

First, MPC rejects the CL imminency requirement.  

Second, a person does not automatically lose the defense because he 
was at fault in creating the necessitous situation.  Instead, MPC 
provides that the defense is unavailable if the actor is prosecuted 
for a crime of recklessness or negligence.  

Third, all forms of necessity qualify: the defense is not limited to 
emergencies created by natural forces, is not limited to physical 
harms to persons or property, and may be employed in homicide 
prosecutions.
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******************************DISTINCTION**************************

Necessity-- Pressure from NATURAL/PHYSICAL forces
Duress- HUMAN threats

Some courts treat duress as a subspecies of necessity, in which 
case, the true distinction between the two is that duress involves 
human threats and the necessity defense involves natural forces.

However, as long as duress is recognized as an excuse defense, the 
difference is imporant.  

The necessity defense applies when a person is faced with CHOICE of 
evils and must then decide whether to commit a crime or an 
alternative act that constitutes a greater evil, and the person 
makes the right choice.  

In contrast, duress applies when the coercing actor's threats 
OVERWHELM the actor's will so that she makes the wrong choice.

The difference has PRACTICAL consequences.  When a person commits 
the lesser of 2 evils, nobody is subject to prosecution because no 
social harm has actually ensued.  

If a person is EXCUSED however, the person who coerced her to commit 
the offense may be prosecuted for the harm caused.