Altar cross

In Paul Turner's Blog by Paul Turner

Q:  Our priest celebrates mass with a second cross on the altar. I wonder why there is the duplication of crosses. GIRM 122 seems to be clear that a single cross is to be used…which is why the processional cross is put away – not visible to the assembly after the opening procession. 
A:  With regard to the altar cross,  GIRM 117 calls for a cross on or near the altar, and you’re right that GIRM 122 presumes that only one cross remains in the sanctuary for the mass. When incense is used, that cross is to be incensed. Pope Benedict’s very public usage of a second altar cross facing the celebrant inspired many priests and bishops to do the same. I find the practice hard to defend from the GIRM.