Q:

Dropdownlist On Second Choose First Selected wont Display:none;

JS
function getData(dropdown) {
    var value = dropdown.options[dropdown.selectedIndex].value;
    if (value == 'emlak'){
    document.getElementById("emlak").style.display = "block";
    }
    if(value == 'vasita'){
    document.getElementById("vasita").style.display = "block";
    }
}

HTML
<select name="kategori" onChange="getData(this);">
     <option value="hat" onClick="">Hat</option>
     <option value="emlak">Shirt</option>
     <option value="vasita">Pants</option>
</select>
<select id="emlak" name="currentList" onChange=";" style="display:none;">
     <option value="hat">Hat</option>
     <option value="emlak">Shirt</option>
     <option value="pants">Pants</option>
</select>
<select id="vasita" name="currentList" onChange="" style="display:none;">
     <option value="hat">Otomobil</option>
     <option value="emlak">Shirt</option>
     <option value="pants">Pants</option>
</select>

When I Choose a Option second time first selected option wont display:none; it should show just one select

A:

Because you are not setting it to none. That would be the reason why.
function getData(dropdown) {
    var value = dropdown.options[dropdown.selectedIndex].value;
    if (value == 'emlak'){
    document.getElementById("emlak").style.display = "block";  //where is the none for the other element?
    }
    if(value == 'vasita'){
    document.getElementById("vasita").style.display = "block"; //where is the none for the other element?
    }
}

Basic idea:
function getData(dropdown) {
    var value = dropdown.options[dropdown.selectedIndex].value;
    var isEmlak = (value == 'emlak');
    document.getElementById("emlak").style.display = isEmlak ? "block" : "none";
    document.getElementById("vasita").style.display = !isEmlak ? "block" : "none";
}

