If you deploy a VanillaDockerApp but don't supply an id: tag then the resulting container takes the name of the image being deployed which is non-unique thereby preventing additional instance of that app yaml being deployed.
For example:
location: my-docker-cloud
services:
- type: docker:nginx:latest
name: nginx
openPorts:
- 80
results in the container called nginx, adding an id: nginx results in a unique container name nginx_<entityid>.