Keyboard problem MacBook Air A1466

Hello everyone! I have a problem with a client's MacBook Air… He brought it to me with a keyboard problem, some keys were typing double letters, like A -> AZ; G -> GM; etc ..

I thought it was a simple problem of a broken keyboard, I replaced it and for a day everything went well. The next morning the 3EDC diagonal stopped working .. Thinking about the defective keyboard I had it postponed, but after a day the 3EDC diagonal stopped working again ...

What can I do?

P.S.: Sorry for my bad english….