Duplication when trying to permute letters (e.g. a-->ä)

I would really appreciate if someone helped me out with this one. To cut it short, every time I try to insert an umlauted vowel (e.g. ü, ä, etc.) or something like “ß”, the program does exactly that but for some reason also inserts the vanilla version of the letter. So I end up with something like “grosß” instead of “groß”. What should I do to fix this?