When a Lead is created with an Email that already exists on an existing Contact, block the Lead creation or automatically link/merge it.

trigger LeadTrigger on Lead (before insert) {
    Set<String> emails = new Set<String>();
    for (Lead l : Trigger.new) if (l.Email != null) emails.add(l.Email);
    Map<String, Contact> byEmail = new Map<String, Contact>();
    for (Contact c : [SELECT Id, Email FROM Contact WHERE Email IN :emails]) byEmail.put(c.Email, c);
    for (Lead l : Trigger.new) {
        if (l.Email != null && byEmail.containsKey(l.Email)) {
            // Option A: block
            l.addError('A Contact with this email already exists.');
            // Option B: l.Existing_Contact__c = byEmail.get(l.Email).Id;
        }
    }
}