The flow creates a new lead although the customer already has an open deal. Can it attach to the existing one?

It does, by default. A lead flow first looks for the customer's contact in the CRM, then for an open deal of that contact, then for an open lead of that contact, then for any open lead that holds the number even without a contact (leads imported from another tool). It binds to the first one it finds and creates a lead only when nothing open exists.

Three situations still produce what looks like a duplicate, on purpose:

• Each number creates its own lead is on in Chat rules: the record is per WhatsApp number, so a customer handled on two numbers has two leads.
• The existing lead is closed. A lost lead can be taken back only inside the re-contact window set in Chat rules; otherwise a new lead is right.
• The phone on the old record is stored in a format that does not match (no country code, a different spelling); fix the phone on the record.

Related

• Which record a flow creates or binds to