guest accounts headless storefront - register afterwards not possible

[Jujuba]

Hello, 

I'm developing a custom headless storefront. 

As my merchant can't afford Shopify Plus and wants to handle login/register I'm syncing customer data with an external database. 

The problem is the following: 

If a customer is purchasing a product as a guest, Shopify will create an account for that customer automatically.

If the same customer wants to register afterwards (using API) Shopify gives an error 'email already taken'.

Is that the expected behavior? 

Is there a solution for this? 

BR