Adding multiple titles/headers with the same content produces invalid markup.

[nicovalencia]

Creating a title/header adds the content of the header to the header markup tag's id attribute.

For example:

# foobar
# foobar

produces:

<h1 id="foobar">foobar</h1>
<h1 id="foobar">foobar</h1>

If setting an id attribute is necessary for #hash linking, it should keep track of duplicates appending a unique identifier. e.g. foobar-1, foobar-2, etc or a timestamp foobar-1389214127156, foobar-1389214127157, etc.

If setting an id attribute is unnecessary, this functionality should be removed altogether.

[nicovalencia]

I'm working on this issue after noticing ids were being added during a bug fix for Ghost.

[nicovalencia]

In addition to incrementing a unique identifier for ids, we should also prefix the id with a namespace. Allowing users who leverage this library as a content editing system opens the door for them to inadvertently match existing style rules based on the header id/content. (Exactly what's happening in Ghost)

I'm proposing a format similar to the following:

## helloworld
## helloworld
## helloworld

<h2 id="section-helloworld">helloworld</h2>
<h2 id="section-helloworld-1">helloworld</h2>
<h2 id="section-helloworld-2">helloworld</h2>

[nicovalencia]

A side-note for this fix/hack:

Users really shouldn't be creating duplicate headers to begin with. Personally, I feel like the library should warn when this happens and not allow for duplicates or any other invalid/non-semantic markup. This would bring an entirely new purpose (markup validation) into the responsibilities of this library, so we should definitely spin off a new thread to deal with that if necessary -- I just wanted to note the underlying issue here.

[pdeschen]

@nicovalencia I think duplicate headers should be allowed and is all fine. What if I have something like

# Chapter 1

## Introduction
...

# Chapter 2

## Introduction
...

Now you're idea about using a suffix counter is an interesting workaround. As an addition: would it make sense to add the original section title as a class (for invariable styling)?

[tivie]

Once we merge the v2 branch, c367a4b will close this. @nicovalencia contribution should be mentioned though, somewhere appropriate.

[nicovalencia]

Makes sense to me -- thank you @tivie!

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