Si hay dos implicit conversion para un mismo tipo el compilador genera un error indicando que las conversiones son ambiguas.

compiler_error.scala

El error que arroja el compilador es:

type mismatch;  found   : List[String]  required: List[(Option[String], String)] Note that implicit conversions are not applicable because they are ambiguous:  both method List2Tuple in object StringImplicits of type (lista: List[String])List[(None.type, String)]  and method List2Tuple2 in object StringImplicits2 of type (lista: List[String])List[(Some[String], String)]  are possible conversion functions from List[String] to List[(Option[String], String)]	Main.scala	/scala_school/src/scala_school	line 27	Scala Problem

what_happen_when_2_implicits_for_the_same_type.scala

package scala_school

object StringImplicits {
  implicit def String2Tuple(value : String) = (None,value) 
  implicit def List2Tuple(lista: List[String]) = lista.map(String2Tuple)
  
}
 
object StringImplicits2{
  implicit def List2Tuple2(lista: List[String]) = lista.map((Some("_SOME_"),_))
}
 
 
object Complex {
   
  
   
  def main(args : Array[String]) : Unit = {
       
       val lista = List("A", "B", "C", "D") 
       
       import StringImplicits._
       import StringImplicits2._
       
       val nuevaLista1: List[Tuple2[Option[String], String]] = lista
       
       println(nuevaLista1)
       
       val nuevaLista2 = lista.map((None,_))
       
       println(nuevaLista2)
       
       
  }
 
}