No, actually, shadow is the wrong term. The correct word is darkness. Darkness is non-existence of light. If non-existence existed, it wouldn't be a non-existence. but we know that darkness exist.
"Infinity is the NON-EXISTENCE of a limit. If a NON-EXISTENCE existed, it would not be a NON-EXISTENCE."
Who says non-existence if it exists, doesn't remain a non-existence. Non-existence of black spots exists in a completely white paper.
Non-existence of protons exists in electrons. Non-existence and existence aren't two opposite QUANTITIES which can be cancelled if they appear together. They are just concepts. Just substitute exist with a different verb, e.g. occur, in your sentence and your problem would be resolved.

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