I am confused with the (revised!) description of GRAPH-SEARCH in AIMA 3/e. By adding resulting nodes to the frontier *only* if not already there (or in the explored set), the search tree "clearly [...] contains at most one copy of each state" (p.77). This description was different in AIMA 2/e.
What about A* search, then, and the Romanian route finding problem in Figure 3.24? There, Bucharest is on the frontier twice, and has to be, because otherwise A* would not find the shortest path.
Thanks for enlightening me,