creating multiple TOCs can cause id conflicts

[zergar]

When I was playing with multiple TOCs for #1532 I've noticed, that creating multiple TOCs in conjunction with headings e.g. named "toc2" can create ID conflicts within the document, leading to some parts not being rendered correctly. I assume, that this can only be fixed by adding some sort of HTML ID management system, but nonetheless.

# h1

## toc2

[toc]

## h2

### h3

### toc3

[toc]

#### h4

#### toc4

[toc]

##### h5

###### h6