I am getting this error in given file while checkout am unable to process any Order.
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It's look like a default magento error.
The bug is discussed here : https://github.com/magento/magento2/issues/4921
The commit for solve this is here : https://github.com/magento/magento2/commit/7fb8a9c0ede398c661f001ac22fa4fb7dc642b2b