Nest the form inside the else condition then the login is handled in a single file
<?php
if(($_POST['username'] == "user1") && ($_POST['pass'] == "pass1"))
{
header("Location:http://www.yahoo.com");
}
else
{ ?>
<form name= "login" method="post" accept-charset='UTF-8'>
<fieldset>
<legend>Login</legend>
<label for="username" >UserName: </label>
<input type="text" id="username" name="username" required="true" />
<label for="pass" >Password:</label>
<input type="password" id="pass" name="pass" required="true"/>
<input type="submit" name="Submit" value="Submit" />
<include'login.php'/>
</fieldset>
</form>
<?php
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
echo "incorrect";
}
}
?>