The Diet of Finland was not unique at all; on the contrary it was a direct continuation of the Swedish Riksdag of the Estates, which had governed both Sweden and Finland since the 15th century. As an autonomous part of the Russian Empire, the Swedish constitution of 1772 remained in force in Finland until independence in 1917. The Russian tsar(s) had vowed to allow the Finns to retain their own laws. This was in part due to the fact that the 1772 constitution in Sweden was a result of king Gustav III's coup d'etat, which had increased the powers of the king greatly at the expensive of the parliament, leading ot the period of 'Gustavian autocracy' until 1809, when the very loss of Finland to Russia caused another Swedish coup which restored parliamentary power. The Swedish Representation Reform of 1866 changed to a more common two-chamber parliament model (unicameral since the 1974 constitution), and the Finnish Diet became unicameral in 1906 and that was continued by the parliament at independence.

The estates (Finnish: säädyt, Swedish: ständer) were not really the same thing as, nor referred to by the same word as later parliamentary chambers (Finnish: kamarit, Swedish; kammare). Although the estates did meet and vote separately and form separate chambers in that sense, the estates - Finland and Sweden had four of them: Nobility, Peasantry, Burghers and Clergy - represented their own specific interests. (but only land-owning peasants were represented, not torppari/torpare - tenant farmers))

They were not there to take a 'second look' at legislature or guard against populist currents and the similar motivations that existed for the later bicameral legislatures. Few laws would've be able to pass if it had required every estate to approve it. There was never any intention to require unanimity among the estates.

The specifics of the Swedish-Finnish version were unique but the concept was not. The French had the Estates General (États généraux), which consisted of their three estates (Nobility, Clergy, Commoners) who met separately, but that institution had no actual legislative power. On the other hand the Scottish parliament had had three estates in a single chamber.

In short Sweden-Finland was far from the only country that had given its medieval estates parliamentary representation. The forms of that organization and not least the amount of power wielded by the assembly varied widely by country and era though.