Post History

# [Pyth], 8 bytes

    qu/Red._

[Try it online!][TIO-ltauip7o]

A list is in a loop if and only if it is the first list seen twice as f is applied repeatedly.

    qu/Red._
     u        Apply the following function,
              until a list is seen twice.
          ._  Take all prefixes of the list
      /R      Count the number of appearance of
        ed    The last element of the prefix
    q         Check if the first repeated is the input

[Pyth]: https://github.com/isaacg1/pyth
[TIO-ltauip7o]: https://tio.run/##K6gsyfj/v7BUPyg1RS/@//9oQx1DHSMgNNYxjgUA "Pyth – Try It Online"