Nested Switch/Case duplicate Default cases problem

/*Program tracks and changes currentPosition.*/

int incomingByte = 0;   // for incoming serial data
int x = 1;              // for incoming numeric data
int currentPosition = 0;  //assume current position starts at zero
int secondByte = 0;     /*The up and down doesn't work. parseInt always returns 0 and x gets turned to 0. 
                        making this variable to try something else.*/

void setup() {
       Serial.begin(115200);     // opens serial port, sets data rate to 115200 bps
}

void loop() 
{
        // take action only on receiving data:
        if (Serial.available() > 0) 
        {
                // read the first incoming byte:
                incomingByte = Serial.read();
                Serial.print("incomingByte = ");
                Serial.println(incomingByte);

                // take action based on the first byte received
                switch (incomingByte) 
                {
                   case 73: // ASCII I - "Idle" command
                        Serial.println("executing case 73 Idle command");
                        Serial.print("currentPosition = ");
                        Serial.println(currentPosition);
                        break;
  
                  case 85:  // ASCII U - "Up" command
                  /*based on Labview code the U and D commands would have a numeric added to the string before that
                  message is sent to the Arduino. So the U command would get the number of positions to move added and become for example
                  U1 or U4 or whatever before being turned into a message.*/
                        Serial.println("executing case 85 Up command"); 
                        if (Serial.available() > 0)
                        {
                          secondByte = Serial.read();
                          Serial.print("secondByte = ");
                          Serial.println(secondByte);
                          switch (secondByte); 
                          {
                            case 49: // ASCII 1
                              Serial.println("executing Up subcase 49: move position up 1");                     
                              currentPosition = currentPosition + 1;
                              Serial.print("currentPosition = ");
                              Serial.println(currentPosition);
                              break;
                            case 50: // ASCII 2
                              Serial.println("executing Up subcase 50: move position up 2");                     
                              currentPosition = currentPosition + 2;
                              Serial.print("currentPosition = ");
                              Serial.println(currentPosition);
                              break;
                              
                            default:
                              Serial.println("invalid up command, currentPosition unchanged"); 
                              break;
                          } //end subcase  
                        }
                        else {
                          Serial.println("No serial data avalable to parse");
                        }
                   
                       break;

                  case 68:  // ASCII D - "Down" command                  
                        Serial.println("executing case 68 Down command");
                        x = Serial.parseInt();  //from original code, always makes x = 0
                        currentPosition = currentPosition - x;
                        Serial.print("currentPosition = ");
                        Serial.println(currentPosition);
                        break;
 
                  case 83:  // ASCII S - "Set Current Position to Zero" command
                        Serial.println("executing case 83 Set command");
                        currentPosition = 0;
                        Serial.print("currentPosition = ");
                        Serial.println(currentPosition);
                        break;

                  /*default:  // for any other input
                       Serial.println("executing case Default - Invalid command");
                       Serial.println("printing a blank space on next line.");
                       Serial.println("");
                       break;
                  */

                }//end case
                Serial.println("Outer case ended"); 
                Serial.println("----------");       

        }//end if
     
}//end void loop