Javascript Validation, only run if first choice is true

I am using the below to validate two drop downs. If the first drop down (choice1) is Yes, I want the code to compare that dropdown with the next (choice2). If Choice1 is No, I don't want anything to happen. I know my sample isn't complete... and for some reason after running it changes both choices to be the same. Forgive me as I am not that great with javascript and still learning. Can someone assist?

function DoCustomValidation()
{
  var frm = document.forms["MyForm"];
  if(frm.Choice1.value = frm.Choice2.value)
  {
    sfm_show_error_msg('They match!',frm.Choice1);

    return false;
  }
  else
  {
    return true;
  }
}

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