lint should check for duplicate tags

[jfirebaugh]

Lint should warn:

• When you use a tag that's "single use only" multiple times (any of the tags that flatten flattens to a non-array value)
• When you use a combination of tags that result in something getting overwritten:
  • A "kind" tag plus explicit @kind
  • A "kind" tag with type plus explicit @type
  • A "kind" tag with name plus explicit @name or @alias
  • Default description plus @description