Fix bug where imported matrix parameter duplicates are not overrided

[benbp]

When importing a matrix from another matrix, the intended behavior is that any duplicate parameter keys favor the value in the matrix doing the import. This PR fixes a bug where this did not happen (instead the imported matrix's value is favored).

[benbp]

@mitchdenny this fixes the issue I mentioned in https://github.com/Azure/azure-sdk-for-net/pull/19191/files#r587679217

[azure-sdk]

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[azure-sdk]

The following pipelines have been queued for testing:
java - template
java - template - tests
js - template
net - template
net - template - tests
python - template
python - template - tests
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See eng/common workflow

[weshaggard]

Looks reasonable

[ghost]

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