To start things off here are three unique heavy tanks from pre & early WW2.
T-39
The T-39 is a heavy multi-turreted breakthrough tank project started in the early 1930's. During it's development eight different layouts were designed, including one with two large turrets each armed with twin 107mm cannons. In 1933 the design was finalised with a single large turret with a 152mm & three smaller 47mm armed turrets. The feasibility of such a large tank and the ever growing costs meant that in 1934 the project was cancelled and work was instead switched to the less complex T-35.
VK100.01 Mammut
In June 1942 Ferdinand Porsche presented the design for the 100t VK100.01 Mammut to Hitler. He liked the design & authorized the construction of a prototype. Soon after he requested some changes be made and over the next eight months the VK100.01 went through several different designs and names until February 1943 when it was finalised as the PzKpfw VIII Maus
Ansaldo GL-4
The Ansaldo GL-4 was an Italian heavy tank project from 1929, very little is known about it's development but the Italian army rejected it due to the complexity and cost. Ansaldo then tried to sell the design to the Soviets who were more interested but preferred to design there own heavy tank.
Note:
There is some argument between historians if the turret was really equipped with two cannons or if the only known image shows a single barrel at both min/max elevation. I couldn't resist the chance to make a WW1 style Mammoth tank tho so I gave it both barrels
