How can I create an Excel command (macro or function) that works like the
VLOOKUP function but sums eventual multiple matches?

Example:

Sheet1 - before execution:
DOG | WHITE
HOUSE | YELLOW
CAT | GRAY

Sheet2 - before execution:
HOUSE | 300
DOG | 780
DOG | 200
BALL | 560

Sheet1 - AFTER execution:
DOG | WHITE | 980
HOUSE | YELLOW | 300
CAT | GRAY |