This error occurs when a pattern matching definition has redundant patterns, i.e., overlapping clauses.

For example, if you have something like:

module Main where
not :: Boolean -> Boolean
not true = false
not _ = true
not false = true

you can notice that the third case of not is redundant, as it is covered by the second case (which, in particular, covers all cases).

So, the compiler will warn:

 Warning in module Main:
 Warning in value declaration f:
 Warning at test.purs line 2, column 1 - line 3, column 1:
      Redundant cases have been detected.
      The definitions has the following redundant cases:

      false

For avoiding this messages, you will need to remove those redundant cases from your definitions. Literals and arrays are not currently checked for redundancy.