Add users` extra data to firestore when they sign in first time with web Firebase firestore

unsubscribeFromAuth = null; //memory leak

  componentDidMount() {
    this.unsubscribeFromAuth = auth.onAuthStateChanged(user => {
      this.setState({ currentUser: user });
      if (user === null)
        return;

      // user data
      const uid = user.uid;
      const photoURL = user.photoURL;
      const name = user.displayName;      

      const userRef = firestore.collection('Users');
      userRef.where('uid','==',uid).get().then((querySnapshot) => {
        console.log("uid size : ",querySnapshot.size);
        // add user extra data when first sign in.
        if (querySnapshot.size === 0) {
          firestore.collection('Users').add({
            uid: uid,
            photoURL: photoURL,
            name: name,
          })
          .then((docRef) => {
            console.log("Document written with ID: ", docRef.id);
          })
          .catch((error) => {
            console.error("Error adding document: ", error);
          });
        }
      });
    });
  }

My code is like this. But when I sign in, querySnapshot.size is 0 and it add user`s data again and again even though there are already user data.

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Like these, they have same uids but signing in made again and again. Is there any problem when I program like this?