The switches contact blister has failed. The keyboard is not really repairable.
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=== Update (03/29/2019) ===
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Yes, this can be confusing! Why does pressing two keys make it work!
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This gets into the mechanics on how a keyboard works. Simply put, its like a spreadsheet a group of columns and rows, in this case the down key has failed so when the neighboring key (electrically) is also pressed the signal gets driven to the decoder logic within the keyboard and it accepts it as the key you wanted.
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Often times a liquid spill is the cause of this type of problem.