az ad group member add is not idempotent

> ### `az feedback` auto-generates most of the information requested below, as of CLI version 2.0.62

**Describe the bug**
I'm running this command from an Azure DevOps pipeline as part of our infrastructure deployment and without additional care these commands fail for subsequent runs.

**To Reproduce**
az ad group member add -g $rg --member-identity $id
az ad group member add -g $rg --member-identity $id

> One or more added object references already exist for the following modified properties: 'members'.

**Expected behavior**
No error.

**Environment summary**

```
azure-cli 2.19.1

core 2.19.1
telemetry 1.0.6

Extensions:
datafactory 0.2.0
azure-devops 0.18.0
resource-graph 1.1.0
application-insights 0.1.5
image-copy-extension 0.0.7
```

**Additional context**
I've run the same command on ubuntu, macOs and Windows. Here's my workaround: https://twitter.com/MRiezebosch/status/1359533376777179137