2DUP 2DROP 2LITERAL
2DUP ( a,a -- a,a a,a )  
ansi .6.1.0380 <CORE>
double-cell duplication, also used to duplicate two items 
 simulate:
   : 2DUP OVER OVER ; ( wrong would be : 2DUP DUP DUP ; !!)

reference: p4_two_dup in ../src/core.c:0448, export CO 2DUP