Merge duplicate comments for same msgid

Extracted comments can get duplicated if you have the same msgid with the same comment:
```po
#. Translators: {{user.name}} is a variable
#. Translators: {{user.name}} is a variable
#: ../bar.html
#: ../baz.html
msgid "Hello {{user.name}}!"
msgstr ""
```

This PR makes sure we only add a comment once:
```po
#. Translators: {{user.name}} is a variable
#: ../bar.html
#: ../baz.html
msgid "Hello {{user.name}}!"
msgstr ""
```