How to fix insert error in table genres?

How can I fix the code below?

He takes the spaces between the commas of the genres ie if I have these genres "action, adventure, romance" it puts so "action, adventure, romance" right? But if I do the reverse order (so "romance, adventure, action") or put more spaces between the commas in an altered or equal way (more or less this way: "action, adventure, romance" or "action, romance, adventure ") is giving error of inserting the same item in the table of MySQL genres.

How can I fix this?

<?php $generoMinusculo = strtolower(trim($_POST["genero"]));
    if ($generoMinusculo) { 
    $generoMinusculo = str_replace(' ,',',',$generoMinusculo);  
    $generoMinusculo = str_replace(' , ',',',$generoMinusculo); 
    $generoMinusculo = str_replace('  ,  ',',',$generoMinusculo);   
    $generoMinusculo = str_replace(' ,  ',',',$generoMinusculo);    
    $generoMinusculo = str_replace('  , ',',',$generoMinusculo);    
    $generos = explode("," , $generoMinusculo); 

foreach ($generos as $item) {
    $Generos = mysql_query("SELECT * FROM 'generos'  WHERE 'genero'='".$item."'");
    $verificar = mysql_fetch_array($Generos);
    $verificarGenero =  $verificar["genero"];
    $generoPostado = $item; 
    if ($generoPostado != $verificarGenero){
    mysql_query("INSERT INTO 'generos' SET 'genero'='".$item."'");
    }elseif ($generoPostado == $verificarGenero){ }
    else { } } ?>