Python: Verify that plugin names are unique.

[stefan521]

Motivation and Context

Addresses #6383

It should not be possible to add two or more plugins with the same name. The .Net version validates that plugin names are unique. I'm adding a similar check in Python.

Description

All the methods to register plugins, such as add_plugins and add_plugin_from_openapi end up in add_plugin so that's where I added the validation.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the SK Contribution Guidelines and the pre-submission formatting script raises no violations
• All unit tests pass, and I have added new tests where possible
• I didn't break anyone 😄

[markwallace-microsoft]

Python 3.10 Test Coverage Report •

File	Stmts	Miss	Cover	Missing
semantic_kernel/functions
kernel_function_extension.py	126	9	93%	45, 47, 338, 358–360, 396, 398, 408
TOTAL	6126	881	86%

Python 3.10 Unit Test Overview

Tests	Skipped	Failures	Errors	Time
1394	1 💤	0 ❌	0 🔥	16.261s ⏱️

[moonbox3]

This was by design since plugins are a Python dictionary, and by default in Python, dictionaries overwrite an existing key if the same key is added.

[moonbox3]

As commented above, this is by design and the Pythonic way of handling it. We're not going to move forward with this change.