Multiple instances breaks WordPress shortcode

I wrote a shortcode that displayed author profiles based on id. For example [user-profile id="1"] would display the profile block defined in user-profile.php for author 1. It worked (even with multiple instances on the same page).

function user_profile( $atts, $content = null ) {
    extract(shortcode_atts(array('id' => ''), $atts));
    include 'user-profile.php';
}

...except the shortcode output was showing up before other entry content regardless of its place in the code. To solve this I added this fix:

function user_profile( $atts, $content = null ) {
    extract(shortcode_atts(array('id' => ''), $atts));
    function get_user_profile() { include 'user-profile.php'; }
    ob_start();
    get_user_profile();
    $output_string = ob_get_contents();
    ob_end_clean();
    return $output_string;
}

...which worked to solve the positioning problem but broke multiple instances of the shortcode. [user-profile id="1"] works but [user-profile id="1"] [user-profile id="2"] breaks it—the page just stops loading at that point.

How can I modify this to allow multiple instances?

Problem solved! I updated the code in user-profile.php code so that it was all PHP and did not use any echos. Then I changed the shortcode function to:

function user_profile( $atts, $content = null ) {
    global $post;
    extract(shortcode_atts(array('id' => ''), $atts));
    include 'user-profile.php';
    return $user_hcard;
}