Add `declaration-block-no-duplicate`

What is the problem you’re trying to solve?

Warn about declaration blocks that can be merged to one:

.selector-a {
  color: red;
}

// rule should warn about this one;
// it can be merged to the previous one
.selector-b {
  color: red;
}

.selector-c {
  color: green;
}

// rule should NOT warn about this one;
// it could be necessary to have this here for specificity
.selector-d {
  color: red;
}

Expected output from autofix:

.selector-a,
.selector-b {
  color: red;
}

.selector-c {
  color: green;
}

.selector-d {
  color: red;
}

What solution would you like to see?

A new rule declaration-block-no-duplicate that warns about consecutive declaration blocks with exactly the same declarations.

[ybiquitous]

I agree that the new rule is only useful for specific use cases and unsuitable for built-in rules.

Or perhaps it would be better to say that it is difficult to create such a rule that would satisfy a generic use case for duplicate declaration blocks.

[ybiquitous]

Again, let me show the rule blueprint below:

• Name: declaration-block-no-duplicate
• Primary option: true
• Secondary options: ignore: ["nonconsecutive-duplicates"]
• Autofixable: No (we can implement later)
• Message: Unexpected duplicate declaration block
• Description: “Disallow duplicate declaration blocks.”
• Section: “Enforce non-stylistic conventions” -> “Duplicate”

The different things from https://github.com/stylelint/stylelint/issues/6524#issuecomment-1352375216 are “Secondary options” and “Section”. Because I think this rule is for conventions rather than “Avoid errors”.