I’ve found a strange problem with cloud-formation roll-backs which don’t automatically remove any Auto Scaling resources you might have setup.
This means then when you next deploy, CloudFormation starts complaining about resources already existing!
To clean these up, you need the following (run from the CLI, using the AWS CLI);
List resources;
aws application-autoscaling describe-scalable-targets --service-namespace dynamodb
From there, de-register (remove) each of the ones which shouldn’t be there;
aws application-autoscaling deregister-scalable-target --service-namespace dynamodb --resource-id "table/myTableName" --scalable-dimension "dynamodb:table:ReadCapacityUnits"
aws application-autoscaling deregister-scalable-target --service-namespace dynamodb --resource-id "table/myTableName" --scalable-dimension "dynamodb:table:WriteCapacityUnits"
That’s it!