Resource redefination should not be allowed

[athompson-r7]

For example the following should throw an error or some kind.

rds_subnet_group 'VPCDBSubnetGroup' do
  description 'RDS Subnet Group for the Razor VPC'
  subnet stack.get('vpc', 'RDSSubnets')
end

rds_subnet_group 'VPCDBSubnetGroup' do
  description 'RDS Subnet Group for the Razor VPC'
  subnet stack.get('vpc', 'RDSSubnets')
end

[erran-r7]

I started looking into this in 2a9d399 and 57786fe. I realised that we call the resource methods twice. Once when loading the stack and once when rendering the template (calling #to_json. Check out the two commits for some more context.

[athompson-r7]

@jmanero-r7 may have some context as to why it is done this way.