Q:

Do Tridion Keyword Keys Need to be Unique?

Could a client use the key value to sort or prioritize Keywords in a given Category?

A:

Yes, but Keyword Keys need to be unique within a Category and the entire BluePrint. Attempting to duplicate a Key (have separate Keywords share a Key) will prevent saving with:

Unable to save item
  Name, Key must all be unique for items of type:
  Keyword within this Category and its BluePrint context. Source or
  sources of conflict: tcm:1-6180-1024.

Keywords by default have:

Value (Title)
Description
Key (non-localizable and must be unique)

Other ways to "sort" Keywords is by naming convention (like Structure Group navigation prefixes) or Keyword Metadata.

