It sounds as if you have two instances of the SCLogin module on your page. If you want to fix the warning now, you'll have to do a template override of the /modules/mod_sclogin/joomlaLogin_vertical.php file and update some styles.
1. In the file you'll replace the line around line 48
<input name="<?php echo $passwordName; ?>" tabindex="0" id="sclogin-passwd" class="input-block-level" alt="password" type="password" placeholder="<?php echo JText::_('MOD_SCLOGIN_PASSWORD') ?>" required aria-required="true">
with
<input name="<?php echo $passwordName; ?>" tabindex="0"class="sclogin-passwd input-block-level" alt="password" type="password" placeholder="<?php echo JText::_('MOD_SCLOGIN_PASSWORD') ?>" required aria-required="true">
2. In your CSS file (/media/sourcecoast/themes/sclogin/sourcecoast.css), you'll need to replace instances of #sclogin-passwd with .sclogin-passwd
I'll create an issue in our tracker to fix this for the next release, but the above should help you workaround it for now.
-Melissa
[DOM] Found 2 elements with non-unique id #sclogin-passwd:
