Receiving Multiple data on Arduino Serial
You send a string "A" 

However, to denote the end, a carriage line (code 13) + line feed (code 10) is sent.

So what is sent is actually an 'A' character followed by a return.
This way you also know when a new line starts

Just strip off the return (13,10) items if you do not need them.

If you do not want to send the return, try sending 'A' (which is the character A, and not the string A.

Update

You can ignore them by:

     if ((incomingByte != 10 &amp;&amp; (incomingByte != 13))
     {
        Serial.print("I received: ");
        Serial.println(incomingByte);
     }


If you only want printable characters, check if you can use the isprint function (isprint function

so it should look like

    if (isprint(incomingByte))
    {
        Serial.print("I received: ");
        Serial.println(incomingByte);
    }