Some object type definitions are repeated in generated markdown files

In the markdown type files, object types are sometimes included multiple times (such as [`FoosResponse` in the plugin integration test](https://github.com/Azure/bicep-types-az/blob/1937f344494cb7c38d0e22454f24edb8e1da7c54/src/autorest.bicep/test/integration/generated/basic/test.rp1/2021-10-31/types.md?plain=1#L35-L41). I believe this is happening because the markdown writer adds [function inputs](https://github.com/Azure/bicep-types-az/blob/1937f344494cb7c38d0e22454f24edb8e1da7c54/src/autorest.bicep/src/writers/markdown.ts#L198) and [outputs](https://github.com/Azure/bicep-types-az/blob/1937f344494cb7c38d0e22454f24edb8e1da7c54/src/autorest.bicep/src/writers/markdown.ts#L201) to the list of types to write w/o pruning duplicates. This does not appear to affect JSON type files.

Inspect src/autorest.bicep/src/writers/markdown.ts around the function input and output handling referenced in the issue, then compare it with the generated integration file linked above. Confirm that duplicate object types are emitted in markdown while JSON files are unaffected. Done means generated markdown type files contain each object type only once.

Written by the indexing model from the issue text.