Hello I was looking for a rule to make sure I don't add duplicated class names in my code but I couldn't find any so I created a custom rule for my project but would be nice to have support for it in this plugin.

Please describe what the rule should do:
This rule can be used to void duplication of class names inside class attributes.

What category should the rule belong to?

• Enforces code style (layout)
• Warns about a potential error (problem)
• Suggests an alternate way of doing something (suggestion)
• Other (please specify:)

Provide 2-3 code examples that this rule should warn about:

<template>
  <!-- ✓ GOOD -->
  <div class="foo"></div>

  <!-- ✗ BAD -->
  <div class="foo foo"></div>
</template>

<template>
  <!-- ✓ GOOD -->
  <div :class="{ 'foo': true }"></div>

  <!-- ✗ BAD -->
  <div :class="{ 'foo foo': true }"></div>
</template>

<template>
  <!-- ✓ GOOD -->
  <div :class="['foo']"></div>

  <!-- ✗ BAD -->
  <div :class="['foo foo']"></div>
</template>

I'd say this rule should check duplicates within each class attribute separately so it should not report cases where a class appears once in class="" and once in :class=""

<template>
  <!-- ✓ GOOD -->
  <div class="foo" :class="'foo'"></div>
</template>

And also should not report duplicates withing each object key/array items or mixed inside :class=""

<template>
  <!-- ✓ GOOD -->
  <div :class="{ 'foo': true, 'foo bar': true }"></div>
</template>

<template>
  <!-- ✓ GOOD -->
  <div :class="['foo', 'foo bar']"></div>
</template>

<template>
  <!-- ✓ GOOD -->
  <div :class="['foo', { 'foo bar': true }]"></div>
</template>

I can help with the PR, tests and documentation as needed :)