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How did you tighten the belt? How much slack do you have?
Can you read the a/c system pressures when the car is running- high and low sides?
The reason I ask about the pressures is that they could be too high. This would cause the compressor to have too much load and make the belt slip on the pully. If you simply added a can of refrigerant, there is a good chance it is overcharged. Since you have a cold/warm duct, there is a chance the expansion valve is not operating properly or the condencer fan is not working well.
The next best step is to check the operating pressures to make sure you are not overcharged. The pressures should read 30/250 on a 70 degree day.