system crash when checkout multiple products

[toreal]

The error messages is as following.

look like MapSet has duplicate keys.

Parameters: %{}
15:36:14.367 [info] Replied cart:1 :ok
15:36:17.178 request_id=prhal3stab9270g36cpl80v9ibjl97o4 [info] POST /checkout/next
15:36:17.187 request_id=prhal3stab9270g36cpl80v9ibjl97o4 [info] Sent 500 in 0繕s
15:36:17.188 [error] #PID<0.636.0> running Nectar.Endpoint terminated
Server: localhost:443 (https)
Request: POST /checkout/next
** (exit) an exception was raised:
** (ArgumentError) error when merging the following Ecto.Multi structs:

%Ecto.Multi{names: #MapSet<[:variant_acquire_stock]>, operations: [variant_acquire_stock: {:changeset, #Ecto.Changeset<action: :update, changes: %{bought_quantity: 1, buy_count: 1}, errors: [], data: #Nectar.Variant<>, valid?: true>, []}]}

%Ecto.Multi{names: #MapSet<[:variant_acquire_stock]>, operations: [variant_acquire_stock: {:changeset, #Ecto.Changeset<action: :update, changes: %{bought_quantity: 1, buy_count: 1}, errors: [], data: #Nectar.Variant<>, valid?: true>, []}]}

both declared operations: [:variant_acquire_stock]

    (ecto) lib/ecto/multi.ex:171: Ecto.Multi.do_merge/3
    (elixir) lib/enum.ex:1755: Enum."-reduce/3-lists^foldl/2-0-"/3
    (nectar) web/workflows/checkout/payment.ex:39: Nectar.Workflow.Checkout.Payment.post_transition/3
    (ecto) lib/ecto/multi.ex:386: Ecto.Multi.apply_operation/5
    (elixir) lib/enum.ex:1755: Enum."-reduce/3-lists^foldl/2-0-"/3
    (ecto) lib/ecto/multi.ex:376: anonymous fn/5 in Ecto.Multi.apply_operations/5
    (ecto) lib/ecto/adapters/sql.ex:508: anonymous fn/3 in Ecto.Adapters.SQL.do_transaction/3
    (db_connection) lib/db_connection.ex:1275: DBConnection.transaction_run/4