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No, your employer can certainly not do that. It is illegal for an employer to pay an employee any less than 1.5 his normal rate of pay for overtime hours, which includes all hours worked in excess of 8 in a day.
If your employer refuses to pay you the overtime that you are owed, you can and should file a wage claim against your employer with the California Department of Industrial Relations, Division of Labor Standards Enforcement.
You can do so using the forms and information available online here:
In addition to the unpaid overtime, you would also be due a waiting time penalty of a day's wages per day you were required to wait to receive the wages you were owed. You may also be entitled to additional damages and fees for your employer's failure to compensate you for the overtime that you have worked.
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