Raise exception when `LocalOperator` finds duplicate sites in `acting_on`

Created by: wdphy16

We should raise an exception when two sites are duplicated in a term of acting_on. For example, it's nonsense to apply a 4x4 matrix on the sites [0, 0], and the user should apply a 2x2 matrix on the site [0] instead. Currently it silently produces unexpected result. It also affects GraphOperator when there is an edge starting and ending at the same vertex in the graph.