Test for duplicate interface implementation instead of declaration

[ShortDevelopment]

Closes #805

PR Checklist

• [x] Created a feature/dev branch in your fork (vs. submitting directly from a commit on main)
• [x] Based off latest main branch of toolkit
• [x] PR doesn't include merge commits (always rebase on top of our main, if needed)
• [x] Tested code with current supported SDKs
• [x] Tests for the changes have been added (for bug fixes / features) (if applicable)
• [x] Header has been added to all new source files (run build/UpdateHeaders.bat)
• [x] Contains NO breaking changes
• [x] Every new API (including internal ones) has full XML docs
• [x] Code follows all style conventions

[ShortDevelopment]

@dotnet-policy-service agree

[ShortDevelopment]

@Sergio0694 Sorry, for pinging you, but I'm kind of blocked by this. Could you please have a look at this PR?