Thank you for posting.
It’s not likely to be a controller or firmware issue, since it has happened suddenly. The firmware wouldn’t have changed, and a sudden problem with the controller would most likely cause the keyboard to stop working altogether.
Double characters are more often caused by radio issues. What happens is that the keyboard issues the key-on message by radio, but the key-off radio message gets lost, garbled or delayed, so the key repeats.
Since this has happened suddenly, I would start by looking for a possible new source of radio interference, such as a new device in the area using the same 2.4 GHz frequency that Bluetooth uses. That includes Wifi, cordless phones, baby monitors, wireless security cameras, other Bluetooth devices, and most wireless keyboards and mice, even that aren’t Bluetooth.
It is also possible that something has happened to the keyboard, such as the internal antenna having a lose connection, or some other problem with the Bluetooth radio chip or wiring.
Could you please answer a few questions about the situation that will help me better understand it?
What is the make and model of the device (phone, computer, tablet, etc.) that you are using the keyboard with?
How far away is the keyboard from that device in normal use?
If you move the keyboard farther away from the device you are using it with, does the problem get worse?