I am practicing building triggers in Salesforce. I have a trigger that creates an opportunity record automatically when an account record's Type field is "Prospect". 
How can I take this trigger further by limiting the number of opportunities created to just one? For instance, every time a person updates the account record to "Prospect", it will create a new opportunity which I do not want since one was created already.

    trigger createOppty on Account (after update) {
        list <Opportunity> Opptylist = new list <Opportunity>();
    
        for (Account o : trigger.new){
            if (o.Type == 'Prospect') {
                Opportunity Oppty = new Opportunity(
                   AccountId = o.Id,
                   Name = 'Edit Opportunity',
                   CloseDate = system.today(),
                   StageName = 'Prospecting');
                Opptylist.add(Oppty);
    
            }
            insert Opptylist;
         }
    }