Ankh-Morpork had dallied with many forms of government and had ended up with that form of democracy known as One Man, One Vote. The Patrician was the Man; he had the Vote.

Somehow always remind me when people mention Athenian democracy

A citizen democracy under dicatorship of the citizen. The citizen had democracy and dictated over those that did not.

If we assume your interpretation is correct, then it relies on the (true) premise that Athens had clear laws about who was a citizen.

That premise does not hold true of “the proletariat”. So, the contradiction remains.

Proof 1

Was Stalin still “proletarian” in Nov 1907, when he was leader of an organised crime ring (he controlled their means of production)? In early 1912, as editor of Pravda (ditto)? In 1918, while directing death squads in Tsaritsyn (ditto)? In 1928 as General Secretary and de facto leader of the USSR (ditto)? From 1941-1953 as Premier (ditto)?

As a kid, he was proletarian. During adulthood, he was increasingly bourgeois - but was still allowed by “Communists” to be a dictator.

So, “dictatorship by the proletariat” is in fact not dictatorship by the proletariat.

=> “Dictatorship by the proletariat” is contradictory. QED.

Proof 2

If you define the proletariat as the disenfranchised, then as soon as the proletariat becomes enfranchised, it is no longer the proletariat.

=> “Dictatorship by the proletariat” is contradictory. QED.

Proof 3

If you define the bourgeoisie as those who ultimately control the means of production, and “dictatorship of the proletariat” as proletarians controlling the bourgeoisie, then in “dictatorship of the proletariat”, the bourgeoisie do not ultimately control the means of production and therefore are not bourgeoisie.

=> “Dictatorship by the proletariat” is contradictory. QED.