Search Query for Multiple Terms In Same Taxonomy

You need to make only one change in the form, change the first line to:

<select name="property_type[]" class="form-control" multiple>

Name of the field has to include [] to be considered as an array when it arrives on the server side. In the query string it will be listed as:

property_type[]=condo&property_type[]=duplex

And, on server side, when you read this use this:

$property_type = (array)$_GET['property_type'];

This can go right at the top just after the $s = $_GET['s']; line,

Make sure to sanitize all the input data before using. More on data sanitizing you can find here: Validating Sanitizing and Escaping User Data