A customer orders on Talabat and also on my own page. Are they one customer in my list?

No. An app order does not create a customer, because the apps hand over a masked number rather than the person's real one. Orders on your own page create and update the customer, earn stamps and can be messaged. So the same person can be a known regular on your page and a stranger on the app. That is one more reason to move regulars across with a link on the receipt. The Orders from delivery apps tab in the docs explains why app orders earn no stamps.