Improve service registration of default member store

[sfmskywalker]

Description

Small change that attempts to add the default member store only if it wasn't added already (potentially with a different implementation)

Purpose

This pull request is a:

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)