Url alias function should mention SEO

Scenario:

I have a node at:
normal path: node/36
url alias: property/test-property

I have a link on this page to a 'subpage' I've created by testing the 3rd argument in the node template to change display a bit.
normal path: node/36/tenants

I wanted to create a link like so:
url alias: property/test-property/tenants

so I have done this:

        $normal_path = $_GET['q'];
        path_set_alias($normal_path . '/tenants', drupal_get_path_alias($normal_path) . '/tenants');
        print l(($num_tenants ? $num_tenants : 0) . ' tenants', $normal_path . '/tenants');

Which works fine for what I've said so far.

Here comes the problem:

Change the title of the node to "Blackwood Apartments".

now the node paths become:
normal path: node/36
url alias: property/blackwood-apartments

and the following error message appears:

user warning: Duplicate entry 'property/blackwood-apartments/tenants-' for key 2 query: INSERT INTO url_alias (src, dst, language) VALUES ('node/36/tenants', 'property/blackwood-apartments/tenants', '') in .../modules/path/path.module on line 112.

It seems to me the problem is this:

FROM path_set_alias() in path.module:

<?php

  else if ($path && $alias) {
    // Check for existing aliases.
    if ($alias == drupal_get_path_alias($path, $language)) {
      // There is already such an alias, neutral or in this language.
      // Update the alias based on alias; setting the language if not yet done.
      db_query("UPDATE {url_alias} SET src = '%s', dst = '%s', language = '%s' WHERE dst = '%s'", $path, $alias, $language, $alias);
    }
    else {
      // A new alias. Add it to the database.
      db_query("INSERT INTO {url_alias} (src, dst, language) VALUES ('%s', '%s', '%s')", $path, $alias, $language);
    }
  }


?>

$alias == drupal_get_path_alias($path, $language) evalutes to FALSE because the existing alias does not match what it should be and thus does not go into the sql that prevents duplicates, but rather the sql to insert a new alias is executed.

Or am I doin it wrong??