error while installing eclipse android plugin
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i have downloaded Eclipse 4.2 IDE for Java Developers and now im trying to install Android plusing and i get this error:

session context was:(profile=epp.package.java, phase=org.eclipse.equinox.internal.p2.engine.phases.Install, operand=null --> [R]com.android.ide.eclipse.adt 20.0.0.v201206242043-391819, action=).

any ideas?
thanks