I used the same keyboard and after several months this happens due to crumbs and stuff getting into the keyboard. I'd turn it upside down and shake. Then vacuum over the keys and see if that corrects the problem. If not, guess you need a new one.
If you Google search (select shopping) you can enter the keyboard name and get a list of all stores selling it. Set the results for "lowest price first". Also try eBay for a new one. I love this MS keyboard but abuse it by eating while I work.
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