Heading pretty poor but I don’t know how to describe this succinctly.

I have a colleague who is trying to turn call centre notes into English but whenever he clicks the end of a word in a cell and starts typing the new text starts inputting 2/3 characters to the left. I.e.
acc
Should be account but end up
ountacc.

Any idea what he he’ll is happening and or how to fix it?