Union types are not a thing in Kotlin.

You may use a sealed class instead.

sealed class CurrentValue<T>(val value: T) {
  class TextualValue(value: String) : CurrentValue(value)
  class NumericValue(value: Double) : CurrentValue(value)
}

Which then you can use exhaustive when expressions (similar to switch in other languages) in order to access the value in a type-safe manner:

fun doSomething(value: CurrentValue<*>) {
  
  when(value) {
    is TextualValue -> value.value // is recognised as a String
    is NumericValue -> value.value // is recognised as a Double
  }

}

If creating a type is way too much for you then you can perform a when statement and treat a parameter based on it's type and perhaps normalize it:

fun parseValue(value: Any?): Double? = when(value){
  is Double -> value
  is String -> value.toDoubleOrNull()
  is Int -> value.toDouble()
  else -> null
}