Duplicate comments should be allowed under different comment_parent

[andy]

Scenario: A post has several top-level comments. A user treating these as tasks wishes to mark them done by replying to each comment, "Done." The user is forced to vary the "done" reply each time.

Duplicate comment detection prevents accidental re-submission. When comment_parent differs, this probably indicates the user's intention to post the same comment a second time.

[andy]

Add comment_parent to duplicate comment check

[scribu]

I could have sworn I opened a similar ticket some years ago.

[SergeyBiryukov]

Related: #9775 (which wasn't actually fixed in [13434] as ticket:9775:12 assumed, since there's still no way to get around the check).

[nacin]

I know these values are "safe" but it would be nice if we also prepared these queries.

[SergeyBiryukov]

21609.2.patch​ prepares the queries.

[nacin]

Since these values are slashed we would need to unslash to prepare. Going to avoid that for now.

[nacin]

In [22229]:

Duplicate comments should be allowed under a different comment parent. props andy. fixes #21609.