Typing replaces text ?

Although it doesnt happen all the time, I have recently been incredibly frustrated to find that when attempting to correct a typing error while composing an email, when I type a letter into a line of text, it deletes the letter immediatly in front of it…
In order to get by, I have to type the new letter and when I hit ‘enter’ and the first letter is deleted from the next word, I have to hit the spacebar enough times to make room for the “damaged” word to be entirely replaced, and then hit backspace to correct the spacing.
Once the corrected letter is placed, every touch of the spacebar deletes another letter from the word in front !

Press “Insert” key on your keyboard and let me know if it helps.