EKS update-config overwrites user details when using multiple roles against the same cluster

I have an EKS cluster where I manage user access using IAM roles. This is roughly what my map roles would look like:

```yaml
- rolearn: arn:aws:iam::123456789123:role/k8s-admin
username: admin
groups:
- system:masters
- rolearn: arn:aws:iam::123456789123:role/k8s-developer
username: developer
groups:
- developers
```

To use `kubectl` with EKS, I need to assume the right role. Therefore, I'll have to provide the role ARN when updating the kubeconfig:

```shell
aws eks update-kubeconfig --name mycluster --role-arn arn:aws:iam::123456789123:role/k8s-admin --alias mycluster-admin
```

So far so good. However, when I try to add my second role to the same kubeconfig...

```shell
aws eks update-kubeconfig --name mycluster --role-arn arn:aws:iam::123456789123:role/k8s-developer --alias mycluster-developer
```

...the user information for the previous config update is overwritten. You can see this in the kubeconfig file:

```yaml
apiVersion: v1
clusters;
- cluster:
certificate-authority-data:
server: https://XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX.YYY.us-east-1.eks.amazonaws.com
name: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
contexts:
- context:
cluster: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
user: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
name: mycluster-admin
- context:
cluster: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
user: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
name: mycluster-developer
current-context: mycluster-developer
kind: Config
preferences: {}
users:
- name: arn:aws:eks:us-east-1:123456789123:cluster/mycluster
user:
exec:
apiVersion: client.authentication.k8s.io/v1alpha1
args:
- token
- -i
- eks
- -r
- arn:aws:iam::123456789123:role/k8s-developer
command: aws-iam-authenticator
env: null
```

Both contexts remain in the config file, but there's only one user which is attached to both contexts. There should be two users with distinct role ARNs.

I haven't verified this from the code, but I'm guessing the user is overwritten because the same user name is used for both updates.