# [Jelly], <s>8</s> 5 bytes

    QL‘=L

[Try it online!][TIO-j8nh9llk]

[Jelly]: https://github.com/DennisMitchell/jelly
[TIO-j8nh9llk]: https://tio.run/##y0rNyan8/z/Q51HDDFuf/4fbHzWt@f8/2lDHTMdYxwQIzXUsY3UUgAJAGAsA "Jelly – Try It Online"

# Explanation

    QL‘=L  - Main link, argument L (a list)   e.g [1,6,3,4,4,7,9]
    Q      - Deduplicated elements                [1,6,3,4,7,9]
     L     - Length                               6
      ‘    - Increment                            7
        L  - Length of the input                  7 ([1,6,3,4,4,7,9])
       =   - Are they equal?                      1

If the output values can be any 2 distinct values, then `QL_L` works, which outputs `-1` for truthy and `-2` for falsey.