Every time I backspace then type in a letter, a second letter dissapears. How do i stop it?

for example, I typed the word "long" but I made a typo and it came out "lnng" so I backspaced one of the n's and put an "o" in its place, but then the second "n" dissapeared so i typed n again and the "g" dissapeared. How do i make it stop!? please help!