"So they have to pay property taxes AND back rent for those five years?"
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Why was the city collecting property taxes on property these people did not 'own'? Wouldn't the payment of property taxes (and the city accepting payment) show that the city believed the people still retained ownership?
There you go again, asking the government not to collect more money that they are supposed to. I have no answer to your question, just the conclusion that these people have been screwed.