const findDuplicate = (str) => {
  const chars = str.trim().replace(" ", "").split("");
  const count = {};
 
  for (let char of chars) {
    const normalized = char.toLowerCase();
 
    count[normalized] = (count[normalized] || 0) + 1;
  }
 
  const isDuplicate = Object.values(count).some((c) => c === 2);
 
  return isDuplicate ? str : null;
};
 
console.log(findDuplicate("asdf"));
console.log(findDuplicate("fdas"));
console.log(findDuplicate("asds"));
console.log(findDuplicate("d fm"));
console.log(findDuplicate("dfaa"));
console.log(findDuplicate("aaaa"));
console.log(findDuplicate("aabb"));
console.log(findDuplicate("aaabb"));
 

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YXNkZgpmZGFzCmFzZHMKZCBmbQpkZmFhCmFhYWEKYWFiYgphYWFiYgo=

asdf
fdas
asds
d fm
dfaa
aaaa
aabb
aaabb