Yours is repeating because you have the echo line inside the goto loop.

The best way to do this would be to use a while loop instead. I have re-written it for you and will attempt to explain the code as well.
Here is the new code.
Code:
alias lista {
  var -s %lnn $nick(#,0,o), %lnp 1
  while (%lnp !> %lnn) {
    var %listops $addtok(%listops,$nick(#,%lnp,o),32)
    inc %lnp
  }
  $iif(%listops,echo -a $v1)
}

And now I will break it down for you.
Code:
alias lista {
  ;Set %lnn as the amount of ops in the room and %lnp as 1
  var -s %lnn $nick(#,0,o), %lnp 1
  ;Start the while loop, it will stop when %lnp is greater than %lnn
  while (%lnp !> %lnn) {
    ;Add the first/next op's name into the %listops token
    var %listops $addtok(%listops,$nick(#,%lnp,o),32)
    ;increase the %lnp variable by one
    inc %lnp
  }
  ;The loop is done now and this next line checks if the %listops token has any names in it and if it does it echos the names.
  $iif(%listops,echo -a $v1)
}


Good luck.

I've gone to look for myself. If I should return before I get back, please keep me here.