If a spell or ability targets Cactarantula more than once, its last ability still triggers only once.

Cactarantula still costs only {1} less to cast if you control multiple Deserts.

Cactarantula’s last ability resolves before the spell or ability that caused it to trigger. It resolves even if that spell is countered.

Once you announce that you’re casting Cactarantula, no player may take actions until the spell has been paid for. Notably, opponents can’t try to change whether you control a Desert.