Pasting same title in html/markdown causes output to be nested

This is an issue that was forwarded to me, but I'm able to reproduce.

If text/html and text/plain contain the same title, the output we return is nested.

Expected output: `[[Same](https://google.com/)](https://google.com)"`
Actual output: `[Same](https://google.com)`

Reproduction: https://codepen.io/tarngerine/pen/ZEZBEpy

Try pressing the `Copy` button on either of the examples and try pasting into any GitHub comment box.

從連結的 CodePen 重現開始，比較將每個範例複製到 GitHub 留言框時發生的情況。追蹤 repository 中的貼上處理，以找出為什麼相同的 text/html 和 text/plain 標題會產生未巢狀的輸出；當重現回傳預期的巢狀 Markdown 連結時，即視為完成。

由索引模型根據 Issue 內容生成。