But it's not that the duopoly is disappearing. It's just that the previous two parties are being eclipsed by two different parities. That's occurred previously in both the UK and US.

The last time it happened in the US was 1856 and its only happened 2x in US history. The US democratic party is the oldest existing political party in the world. For reference, the UK is actually only about 90 years older than the Democratic party.