L = [
    {"id": 1, "name": "john", "age": 34},
    {"id": 1, "name": "john", "age": 34},
    {"id": 2, "name": "hanna", "age": 30},
]

>>> {v["id"] for v in L}

{1, 2}

参考：
https://stackoverflow.com/questions/11092511/list-of-unique-dictionaries