More's Utopia may be read both as an allegory and as a satire : an allegory of an ideal / perfect state, and a satire on the impossibility of the ideal state. 'Utopia' is made up of 'ou'(no) and 'topos'(place). In that sense, the name refers to 'no place'. but its homophone 'Eutopia' is made up of 'eu'(good) and 'topos'(place). In that sense, the name refers to 'good place'. That is to say, if the ideal commonwealth is 'good place', it is also 'no place'.
More's island is fictional and so is its social-economic-legal system. Wars are discarded, but there are the mercenaries to be sent for the warring enemies. There are slaves and ideological suppression. Since there are few laws and lawyers, there is hardly any significance of legal justice.
More's tract thus suggests the humanist perception of an ideal society on one level, but on another level it may also be understood as an authoritarian system which absurdly and mechanistically conjures up a 'no place'. The all good is only too good to exist.
The notion of utopia is always to be viewed against the backdrop of dystopia, and the dynamics of the two have shifted depending on the historical period and cultural context. If by dual role you mean a standard by which one necessarily fails as well as a standard to which to aspire, then of course utopias function in in two ways. But there is the dystopia in there too, and the relation in each context between utopia and dystopia is perhaps more important and more useful than a focus on the dual role of utopias.