Keyboard problem - on left side, striking a single key causes two activate

My phone (which I've had about 18 months) has developed a weird fluke on some keys on the left side of the keyboard:

• If I press 1 or 2, it types "12"
• If I press q or w, it types "qw"
• If I press a or s, it types "sq"
• If I press Shift or z, it types "Z" and then reverts to non-capitalization

I've tried turning it off & back on, and removing the battery then putting it back in, but nothing seems to reboot the keyboard. Any idea of why this might be so?