-- in.cue --
import "list"
x: list.UniqueItems
x: [string, ...string]
x: ["x","x"]
-- out/eval --
Errors:
x: invalid value ["x","x"] (does not satisfy list.UniqueItems)

Result:
(_|_){
  // [eval]
  x: (_|_){
    // [eval] x: invalid value ["x","x"] (does not satisfy list.UniqueItems)
    0: (string){ "x" }
    1: (string){ "x" }
  }
}
-- out/compile --
--- in.cue
{
  x: 〈import;list〉.UniqueItems
  x: [
    string,
    ...string,
  ]
  x: [
    "x",
    "x",
  ]
}