Stock and allocated are incorrectly updated due to multiple same orderline in one order

[simpian]

Describe the bug
A clear and concise description of what the bug is.

current apples in stock: 50
currently being allocated: 20
order

1. apple x 10, delivery date 10/10/2021 (in memory product variant with allocated: 20)
2. apple x 5, delivery date 10/12/2021 (in memory product variant with allocated: 20)

because apple are considered as two order items due to different delivery dates, however, they are the same product variant, current code will use the allocated number that is stored in memory to calculate new stock.

after the first loop:
the allocated number is 20 + 10 = 30 allocated,
however, the second loop will cause a bug because it is still using the old allocated number in memory to calculate new allocation: 20 + 5 = 25 instead of 30 + 5 = 35, the correct allocated number should be 20 + 10 + 5 = 35.

To Reproduce
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Expected behavior
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Environment (please complete the following information):

• @vendure/core version:
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Additional context
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