Copy and paste bold text results in a double bold text in Firefox and IE

Steps to Reproduce

1. Create a new page using any browser
2. Type a text bold
3. Select the text and bold the text
4. Select the bold text and copy it
5. Place your cursor after bold text (besides letter d)
6. Ensure that the bold icon is turned on/selected and then press ENTER to go to the second paragraph
7. Paste the text
8. Copy the text that you pasted in the previous step
9. Paste the text into the third paragraph

Expected Results

Bold texts should be displayed normally in Firefox. There should be no difference between the first, second, and third bold texts.

Actual Results

Instead, the texts are displayed in Firefox or IE as the following:

The second bold text is stronger than the first one

In Chrome, the texts are shown without any difference.

Environment Information

Chrome 26, Firefox 19, IE 9
Tested on Confluence 4.3.x and 5.0.x

Findings

By inspecting the Storage Format, we can see that the <strong> tags are used twice in the second paragraph, and thrice in the third paragraph.

<p>
  <strong>bold</strong>
</p>
<p>
  <strong>
    <strong>bold</strong>
  </strong>
</p>
<p>
  <strong>
    <strong>
      <strong>
        <strong>bold</strong>
      </strong>
    </strong>
  </strong>
</p>

Workaround

Ensure that the bold icon in the editor is not selected/turned on when pasting a bold text