php foreach() w/ MySQL WHERE w/ while{} returning only the last array() item

You do not need the foreach in the while loop, here:

$query_seminars = mysql_query("SELECT id, name, UNIX_TIMESTAMP(moment) FROM seminars WHERE id={$s}");
while ($sem_row = mysql_fetch_assoc($query_seminars))
{
    $names .= "-- " . $sem_row['id'] . " " . $sem_row['name'] . date('D, F j, Y,     g:i a', $sem_row['moment']) . "<br />";
}

Also to use named indexes you use mysql_fetch_assoc, not mysql_fetch_array. (Already in above code)

The problem is not that MySql is returning duplicate (actualy tri-plicate) items. The problem is that you are iterating over each record with your foreach, as others pointed out.

One usual idiom used with MySql is:

$sql = "SELECT id, name, UNIX_TIMESTAMP(moment) FROM seminars WHERE $s=id";

if ($q = mysql_query($sql)) {
  while ($f = mysql_fetch_assoc($q)) {
    //($f is an associative array of the record value, 
    //with keys for each field)
    //... do something with $f
    echo "--" . htmlspecialchars($f['name']) 
      . date('D, F j, Y, g:i a', $f['moment']) 
      . "<br /><br />";
  }
} elseif ($e = mysql_error()) {
//do something with the error message
//...
}

You are seeing "duplicates" because you are using a foreach inside the while loop to iterate by each field of the returned record. Since your query is returning records with three fields each (id, name and moment), you are processing each record three times.