Unable to add default route (0.0.0.0/0) over Transit Gateway attachment

SYMPTOM

Unable to add a default route (0.0.0.0/0) via Transit gateway with error:
"This subnet has already been added to this route table. Learn more about VPC routes."

CAUSE

Due to Cloudhub's architecture, Cloudhub VPCs need to have a default route (0.0.0.0/0) going over an AWS Internet Gateway (IGW), which will take precedence over a propagated default route. Removing said IGW is not possible as it is required to operate and manage the environment as part of our service offering. Moreover, doing away with it could not only cause adverse effects and disconnects between the management plane and the runtimes, but it would also make it impossible to ensure our service offering's SLAs.