Question about "match_dup"

[fishbaoz@hotmail.com]

What is the exact meaning of "match_dup"?

the following is extracted from "mips.md"

(define_expand "movsicc"
  [(set (match_dup 4) (match_operand 1 "comparison_operator" ""))
   (set (match_operand:SI 0 "register_operand" "")
 (if_then_else:SI (match_dup 5)
    (match_operand:SI 2 "reg_or_0_operand" "")
    (match_operand:SI 3 "reg_or_0_operand" "")))]
  "mips_isa >= 4"
  "
{
  gen_conditional_move (operands);
  DONE;
}")

Now?
what is the meaning of "match_dup" in the machine description above?
what does it "duplicate"? Operand(4)? where is the original Operand(4)?