Markdown parsing should check for possible duplicate headers rather than just take the first one found

[fsuits]

Describe the bug

The markdown parser currently queries the nodes for a matching key and returns the first one found. This could be a problem if there are multiple matching nodes when only one was expected. New functions have been added to return all matching keys, so cases where only one match is expected should check for a possible duplicate and either error or warn of the ambiguity.