Internal pull-up resistors used with external pull-ups

I'm looking at the example at http://energia.nu/Tutorial_Button.html

In the circuit, an external pull-up resistor is used.  Then the code reads:

 // initialize the pushbutton pin as an input:
  pinMode(buttonPin, INPUT_PULLUP);     

The Energia reference page http://energia.nu/PinMode.html doesn't explain what INPUT_PULLUP does, but it seems reasonable to think it enables the processor's internal pull-up resistor.

Are there any issues with enabling the processor's internal pull-up resistor and also use an external pull-up resistor, as is the case here?