'credential reset -append' not idempotent

### Describe the bug

When running the command again for the same certificate it will import a duplicate.

### Related command

`az ad app credential reset --id $appId --cert "@$cerPath" --append`

### Errors

None

### Issue script & Debug output

N/A

### Expected behavior

I propose that if a certificate with the same thumbprint already exists in the app, a duplicate should not be created - regardless of the `--append` switch.

### Environment Summary

azure-cli 2.84.0

core 2.84.0
telemetry 1.1.0

Dependencies:
msal 1.35.0b1
azure-mgmt-resource 24.0.0

Python location 'C:\Program Files\Microsoft SDKs\Azure\CLI2\python.exe'
Config directory 'C:\Users\gsmol\.azure'
Extensions directory 'C:\Users\gsmol\.azure\cliextensions'

Python (Windows) 3.13.11 (tags/v3.13.11:6278944, Dec 5 2025, 16:26:58) [MSC v.1944 64 bit (AMD64)]

Legal docs and information: aka.ms/AzureCliLegal

Your CLI is up-to-date.

### Additional context

_No response_