Hi @gegraha ,
Does it only happen in emails or also when you use different apps requiring text input e.g. browsers?
Has anything been spilled on the keyboard?
Do you get multiple entries of the # 6 when you press the #6 key just once when using a text type app?
Visually check the key to see if it appears to be stuck in a half operated position. If it is then perhaps it is just the vibrations caused by using the keyboard that may be causing the contacts to operate when they shouldn't.
You can try using either a Bluetooth keyboard or a USB keyboard (connected via a suitable OTG - On The Go - adapter) to verify if the problem is hardware or software related. If there is no problem when using the other keyboard then there is a fault with the RCA keyboard.
It is usual to replace keyboards as you can cause more problems than you had when trying to repair them.
Here is a link to a supplier of the keyboard. It is not a recommendation to use them, it is shown to give an idea of the price. The may be others that suit you better.
Is the tablet/keyboard still covered by the manufacturer's warranty at all?
If you think so, first verify the purchase date on the sales receipt or proof of purchase document and if the warranty period is still valid consult the warranty statement (usually found in the User Guide) as to what to do to make a claim for a warranty repair or replacement.