Further simplification of weekly_amount validation logic
Further simplification of weekly_amount validation logic

* The two NOTs in the first condition cancel each other out
* Applied De Morgan's laws on the second condition i.e. "not (A or B)" is the
same as "(not A) and (not B)".
* Also "not (A > B)" is "A <= B" and "not (A < B)" is "A >= B"

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