<?php
defined('_JEXEC') or die;
jimport('joomla.plugin.plugin');
class plgSystemPlugin extends JPlugin {   
    public function onContentPrepareForm($form, $data) {
	if (!($form instanceof JForm))
	{
	    $this->_subject->setError('JERROR_NOT_A_FORM');

	    return false;
	}

	// Check we are manipulating a valid form.
	$name = $form->getName();

	if (!in_array($name, array('com_admin.profile', 'com_users.user', 'com_users.profile', 'com_users.registration')))
	{
	    return true;
	}
		
	$form->setFieldAttribute('fieldname','class','hello','com_fields');
    }
}

This is fun - it occurred to me to trigger a log when the FieldsHelper::prepareForm method was executed. Adding error_log('prepareForm'.microtime()); to the method gave this result in the frontend:

PHP message: prepareForm0.75458500 1509635643
PHP message: prepareForm0.77258000 1509635643

The FormHelper::prepareForm method is being executed twice.

I get this log in the backend:

PHP message: prepareForm0.28052000 1509636249

Something in the frontend is causing FieldsHelper::prepareForm to execute twice.

debug_backtrace shows that in the frontend it's being executed in:

        (
            [file] => /var/www/html/jcb/libraries/joomla/event/event.php
            [line] => 70
            [function] => onContentPrepareForm
            [class] => PlgSystemFields
            [type] => ->
        )```
and in
```Array
        (
            [file] => /var/www/html/jcb/components/com_users/views/profile/view.html.php
            [line] => 50
            [function] => getForm
            [class] => UsersModelProfile
            [type] => ->
        )```

But in the backend, it's only being executed in:
```Array
        (
            [file] => /var/www/html/jcb/libraries/joomla/event/event.php
            [line] => 70
            [function] => onContentPrepareForm
            [class] => PlgSystemFields
            [type] => ->
        )```

It could be related to this PR #18211. If yes, please test and submit your result.

Thanks @Quy - that's the exact change I just tested! Commenting out those two lines resolved the issue double processForm issue - but I was beginning to test for side effects.

I'll keep an eye on that issue while I continue to investigate mine. If I determine that to be the fix, I'll close this issue.

My testing is complete, PR #18211 solves this issue. I'll drop a note over there. Thanks again @Quy for making that connection!