[Bug]: Handle same name importer marker

[rytswd]

🔖 Version

v0.0.1-rc4

📌 What happened?

When the same Improter marker name is used multiple times, Importer won't be able to process correctly.
This should be an error, but with the current version, this does not give you any error, and would try to process and end up with incorrect data.

Example:

# Test Markdown

<!-- == imptr: some_importer / begin from: ./some-data.md#1~2 == -->
<!-- == imptr: some_importer / end == -->

<!-- == imptr: some_importer / begin from: ./another-data.md#33~38 == -->
<!-- == imptr: some_importer / end == -->

🪵 Relevant log output

No response

🔗 Reference

No response

[rytswd]

This is fixed with #39