Cookbook: Associating multiple values with each key in a dictionary

[Egbert Bouwman]

On page 9 of the free chapter 01 of their cookbook
alex and associates present a way to associate multiple values 
without duplications to a dictionary:
    d2 = {}
    d2.setdefault(key, {})[value] = 1
This produces nested dictionaries with unique keys,
however with values that I don't need.

I prefer a solution with lists, as in the one with duplications allowed:
    d1 = {}
    d1.setdefault(key, []).append(value)
    
This function creates a multivalued list without duplications:

    def keyplus(dic,key,value):
        dic.setdefault(key, [])
        if value not in dic[key]:
            dic[key].append(value)
            
but now I have a feeling that some one-liner is possible. If not, 
don't we need then a list method 'append_if_not already-there' ?
            
            



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