查找并删除字符串中的所有重复项，除非它们重复奇数次

I have been trying to solve this task for probably more than an hour already. I need to remove all the duplicates from a string but the tricky part is that if a letter is duplicated and odd number of times, one copy of that letter should remain in the final string. For example a string of I have been trying to solve this task for proba