I had the issue with the doubling with the b key as well.

Mine was out of warranty, so I fixed it myself.

Failed: compressed air Failed: cleaning under the key.

Success (but not recommended) I removed the key, then pulled off the glued membrane. Cleaned all with 90% alcohol. Took a new membrane (but you might be able to use old) and used E6000 and a toothpick to glue it back. Replaced butterfly and key.

Not a good solution, but it worked.

It looked like the contacts were getting coated and not making good contact? Perhaps heat?

Best of luck.