[bug-recutils] Field expressions allow duplicate values, but should not

[Mampir]

It's written in the manual that filed expressions should not return
duplicated values, however the utilities do return duplicates.

An example, similar to to one given in the manual:

  # -*- mode: rec -*-
  
  %rec: Maintainer
  %type: Name line
  %type: Email email
  
  Name: Recht Sels
  Email: address@hidden
  Email: address@hidden
  Email: address@hidden
  
  # End of maintainers.rec

Selection command with output:

  $ recsel -t Maintainer -p Email[0],Name,Email maintainers.rec
  Email: address@hidden
  Name: Recht Sels
  Email: address@hidden
  Email: address@hidden
  Email: address@hidden

According to the manual, the result of the FEX should be as like was
"Email[0],Name,Email[1-2]".  The output should be:

  Email: address@hidden
  Name: Recht Sels
  Email: address@hidden
  Email: address@hidden