Email form input check against existing user emails

I have sign-up/registration form and I’m trying to check if the email matches the email of existing user records. I’m trying to avoid displaying the default JavaScript alert() pop-up for “This email already in use” and deal with displaying a message using a Custom Workflow.

First of all you might have issues with this workflow as steps can run in parallel. They are not guaranteed to execute in sequence as you see Step 1 → 2 → 3 etc…
So, your Step 3 might execute before your Step 2 where you terminate the workflow.

One way to make sure it runs in sequence is to trigger several custom workflow that wait for each other’s answers to run.

You should use a custom workflow to check if the email already exists, this workflow will return a yes/no value and then you can use this returned value as a condition to trigger a custom_signup workflow.