YAML.load("x: 3")         # Dict{Any, Any} with 1 entry: "x" => 3
YAML.load("x: 3\nx: 4")   # Dict{Any, Any} with 1 entry: "x" => 4     (<-- this is wrong)

@kwdef struct UniqueKeyDict{T1, T2} <: AbstractDict{T1, T2}
    dict::Dict{T1, T2} = Dict{T1, T2}()
end

function Base.setindex!(ukd::UniqueKeyDict, value, key)
    haskey(ukd.dict, key) && error("Key $key already exists in dictionary.")
    ukd.dict[key] = value
end

YAML.load("x: 3")                                               # Dict{Any, Any} with 1 entry: "x" => 3
YAML.load("x: 3\nx: 4")                                         # Dict{Any, Any} with 1 entry: "x" => 4
YAML.load("x: 3"; dicttype=UniqueKeyDict{Any, Any}).dict        # Dict{Any, Any} with 1 entry: "x" => 3
YAML.load("x: 3\nx: 4"; dicttype=UniqueKeyDict{Any, Any}).dict  # throws an error