Dictionary has same key more than once

New to Julia
3

This may be caused by mutation of keys. I made a simpler example based on yours:

julia> mutable struct MyRef{T}  val::T  end
julia> Base.hash(a::MyRef, h::UInt) = xor(a.val, h)
julia> Base.:(==)(a::MyRef, b::MyRef) = a.val == b.val

julia> d = Dict{MyRef{Int}, Symbol}()
Dict{MyRef{Int64}, Symbol}()

julia> x = MyRef(0); y = MyRef(0); d[x] = :x; d[y] = :y; d # y replaces x
Dict{MyRef{Int64}, Symbol} with 1 entry:
  MyRef{Int64}(0) => :y

julia> y.val = 1; d[x] = :x; d # mutate y, now add x
Dict{MyRef{Int64}, Symbol} with 2 entries:
  MyRef{Int64}(1) => :y
  MyRef{Int64}(0) => :x

julia> y.val = 0; d # mutate y back, now same key as x
Dict{MyRef{Int64}, Symbol} with 2 entries:
  MyRef{Int64}(0) => :y
  MyRef{Int64}(0) => :x

julia> d[x], d[y] # uh oh
(:y, :y)
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