Hello,
My renter left town for one month. She hasn't paid her rent for this month. On top of that, she left a lot of people living there (5 adults and 3 kids). It was a shock for me when I saw the duplex occupied and destroyed by these people. However, I called the police and accused those people of trespassing. Nevertheless, she called the police and told that those were her guests and the police didn't intervene anymore. What can I do to get rid of those people?
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Evict them, for destroy property, other violations of the lease you have.

The sooner you start a formal eviction according to the laws of your state, the faster this will happen.

It depends on the particular law of whichever state you are in and the terms of the lease, but, in general, you will have to give them a written notice and, if they fail to go by then, you would sue them for eviction.

When was her rent due? Is she on a lease or month to month?

If she is violating the terms of the lease you can send her a pay or quit notice. Such a notice will give her a number of days (depending on local laws) to repair the breach in the lease (unpaid rent, property damage, unauthorized occupants) or vacate the property. If she does not comply within the deadline, then you file for eviction in housing court.

You should have sent her such a notice as soon as her rent was late.