Describe the bug
If the cluster and user names of multiple KubeConfig contexts are duplicated, the final merged kubeconfig will have the wrong content!

To Reproduce
Steps to reproduce the behavior:

1. Prepare three kubeconfig files, such as .kube/cluster1,kube/cluster2,kube/cluster3
2. They have same context:

contexts:
- context:
    cluster: kubernetes
    user: kubernetes-admin
  name: kubernetes-admin@kubernetes

3. use kubecm merge them:

touch ~/.kube/config
kubecm add --file .kube/cluster1
kubecm rename # cluster1
kubecm add --file .kube/cluster2
kubecm rename # cluster2
kubecm add --file .kube/cluster3

4. Result: .kube/cluster3 add done, but context