I gave 1st and last month/depoist rent when I moved in. Lease is month to month now with 30 days notice required. Do I still have to pay last months rent if landlord already has from beginning of lease? What happens if I don't pay it?

I don't know the wording in your lease but your last month's rent is exactly that - full rental for the last month you live there.

No, you do not owe additional rent.

"1st and last month/depoist rent " this is a combination of several terms which should not be considered the same. We would have to see your lease but if your rent, for example , was $1000 per month and you gave the LL $2000 up front most LLs would consider this as first months rent and $1000 security deposit which should not be considered the final months rent. The LL can hold the last $1000 until he is sure there is no damage to the unit. It all depends on the wording of the lease.