Difference between do and subscribe in rxjs

Question

In this JS Bin I have some observables that load a mocked paginated data source. getPageFromServer loads a page of data from the server starting at the given index:

function getPageFromServer( index ) {

  index = index || 0;

  var values = [];
  var nextIndex;

  for(var i = 0; i< 3; i++) {
    var newValue = index + i;
    if(newValue < 10){
      values.push( newValue );
      nextIndex = newValue + 1;
    }
    else {
      nextIndex = undefined;
    }
  }

  data = { values: values, nextIndex: nextIndex };

  return Rx.Observable.return(data).delay(500);
}

getPagedItems then loads the pages and concats the multiple observables into one so that we just get an observable of all the values from all the pages:

function getPagedItems(index) {
  return getPageFromServer(index)
    .flatMap(function (response) {
      var result = Rx.Observable.from(response.values);

      if(response.nextIndex !== undefined){
        return result.concat( getPagedItems(response.nextIndex) );
      } else {
        return result;
      }

  });
}

This gives me my expected result:

"page received: 1"
"page received: 2"
"page received: 3"
"page received: 4"
"page received: 5"
"page received: 6"
"page received: 7"
"page received: 8"
"page received: 9"
"complete"

However if I add a do to the end of getPagedItems:

function getPagedItems(index) {
  return getPageFromServer(index)
    .flatMap(function (response) {
      var result = Rx.Observable.from(response.values);

      if(response.nextIndex !== undefined){
        return result.concat( getPagedItems(response.nextIndex) );
      } else {
        return result;
      }
  })
  .do( function ( result) { console.log( "do: " + result); } );
}

I get each value the number of times equal to the number of observables we have created in the flatmap:

"starting..."
"do: 0"
"page received: 0"
"do: 1"
"page received: 1"
"do: 2"
"page received: 2"
"do: 3"
"do: 3"
"page received: 3"
"do: 4"
"do: 4"
"page received: 4"
"do: 5"
"do: 5"
"page received: 5"
"do: 6"
"do: 6"
"do: 6"
"page received: 6"
"do: 7"
"do: 7"
"do: 7"
"page received: 7"
"do: 8"
"do: 8"
"do: 8"
"page received: 8"
"do: 9"
"do: 9"
"do: 9"
"do: 9"
"page received: 9"
"complete"

Please explain to me why "do" is firing multiple times but "onNext" in the subscription is only firing once.

Many Thanks

Answer 1

I figured out this answer about 5 minutes after I posted the question!

The reason that the do fires multiple times is because I am calling the loadCommentThreads recursively so each nested call to loadComment threads has a do in the pipeline before the message returns to the parent calling function instance.