node.js readline - write data appears in input

I've got a very basic readline implementation which seems to have a problem that when I write the written data appears in input and raises a 'line' event. This is all in a standard linux rxvt window.

var rl  readline.createInterface({
    input: process.stdin,
    output: process.stdout,
//    terminal: false   <--- setting true/false makes no difference
});

rl.on('line', function(line) {
    var inp = line.trim();
    console.log('line event');
    switch(inp) {
        case   ....
    default:
       console.log('Unknown command: ' + inp + '\n');
    }
});

And then rl.write(string);

from socket i/o. The write raises 'line' event and the written data apears as an unknown command.

If I enter data on terminal it raises 'line' event as expected and works OK.

Clearly data written shouldn't appear on input and asking what might be the reason for this.

Thanks