I gave my 30 days notice to leave my apartment 1 month before my lease was up. The management co. agreed to it but demanded the last months rent be paid. Are they allowed to take my money for that month I won't even be there and then rent it out to someone and take their money for that month as well? In Oregon I know this is not legal but I need to check for CA.

No, they're not entitled to collect double rent. If they are able to get a new tenant into the property for the entire last month of your term then they have to refund that rent payment to you.