This is pretty obvious when you think about it, but still neat.

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Example:
--------

    This is a link to [RFC 1796][1796].

    So is [this][rfc1796].
    
    [1796]: https://tools.ietf.org/html/rfc1796    
    [rfc1796]: https://tools.ietf.org/html/rfc1796    

Result:
-------

This is a link to [RFC 1796][1796].

So is [this][rfc1796].

[1796]: https://tools.ietf.org/html/rfc1796
[rfc1796]: https://tools.ietf.org/html/rfc1796

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The practical upshot is that you don't have to worry about numerical sequencing or having to rearrange things if edits are made at a later date. Definitely useful for tag wiki stuff, or any other giant blob of RFC hyperlinks that you're throwing together.