Allow multiple references to the same footnote

[jakov]

I just noticed, that you can't use the same footnote reference twice. This behaviour is actually the same as in PHP Markdown Extra, but it has also been discussed over there:

    - refer to footnote A[^1]
    - refer to footnote B[^2]
    - refer to footnote A again[^1]

[^1]: footnote A
[^2]: footnote B

The "refer to footnote A again[^1]" line doesn't get linked to footnote A.

[tanakahisateru]

For compatibility with PHP Markdown I want to retain this issue as not closed.
I will work for it if PHP version would be fixed in the next release.