Well unfortunately, you would have to do one of two things (at least of what I can think of). Either you would have a variable, that would track the use of a character. If a character is used, then an associated variable would be set to true. The next time they press the character, if when checking that variable and true is returned, then the keyboard entry is ignored as a duplicate.

The other way would be to compare the keyboard entry (each and every entry) against the text in the input field. If a character in the field matches the keyboard entry, then it is ignored (user notified) otherwise it is inserted at the end of the text group.

Of course, there are much smarter people out there and they may have better options, these are just what jumped in my head at the moment.