deployment of multiple subnet within same vNet failing in subsequent deployment

101-vnet-two-subnets (https://github.com/Azure/azure-quickstart-templates/blob/master/101-vnet-two-subnets)

### Issue Details
When we are deploying the subnets for different environment in the same vnet they are failing. We have tried another template https://github.com/Azure/azure-quickstart-templates/tree/master/101-subnet-add-vnet-existing to solve it by using two template (1 for only vnet and other for the subnet) which is failing as well.

### Repro steps (*if necessary, delete otherwise*)
1. Create a vNet named (DevTest)
2. Create some subnet in the vNet for the 'Dev' environment. All subnets suffixed with -Dev (i.e. Web-sNet-Dev for web and Data-sNet-Dev for data)
3. Create a VM which has a network card attached, network card is created in one of the subnet created for the 'Dev' environment.
4. Redeploy the subnet again for the test environment where new subnets (suffix with -test) should be created within same vNet.
5. Template deployment fails as it tries to delete existing subnet before deploying the new ones.
6. If you use two template, one for the vNet and other for subnets (101-subnet-add-vnet-existing), it fails as well and complains that another deployment is going on.

Ideally ARM template deployment should be idempotent, we have specified incremental option as well but no help.

Start with the 101-vnet-two-subnets template and compare it with 101-subnet-add-vnet-existing. Reproduce the Dev and Test subnet deployments in the same DevTest vNet using incremental mode, then determine whether the failure is caused by subnet replacement or a concurrent deployment; done means the reported redeployment scenario succeeds without removing existing subnets or conflicting deployments.

Written by the indexing model from the issue text.