componentDidMount getting called multiple times in redux connected component

Question

I have a reactjs+redux app in which app.js, the first component to be mounted is given below:

//all imports here
class App extends React.Component {
    
    constructor(){
        super();
        this.state = {
            loginState : false,
        }
    }

    componentDidMount() {
        firebase.auth().onAuthStateChanged((user) => {
            if (user) {
                console.log(user);
                if(user.displayName&&user.photoURL&&user.email)
                    this.props.dispatch(login(user.displayName, user.photoURL, user.email));
                else
                    this.props.dispatch(login(user.email.split("@")[0], "", ""));
                this.setState({
                    loginState: true
                });
            }
            else{
                this.props.dispatch(changeLoading());
            }
        });
    }
    logout = () => {
        
        firebase
            .auth()
            .signOut()
            .then(() => {
                this.setState({
                    loginState : false,
                });
                this.props.dispatch(logout());
                this.props.history.push('/login');
            })
            .catch((err) => {
                console.log(err);
            });
    };
    render() {
        
        return (
            <>
                <Switch>
                    {this.props.userName?(<Route
                        exact
                        path="/"
                        component={() => <Homepage userName={this.props.userName} />}
                    />):(<Route exact path="/" component={Loading} />)}
                    <Route exact path="/login" component={Login} />
                    <Redirect to="/" />
                </Switch>
            </>
        );
    }
}

const mapStateToProps = (state) => {
    return {
        isLoggedIn: state.userState.isLoggedIn,
        userName: state.userState.userName,
        email: state.userState.email,
        photoURL: state.userState.photoURL
    };
};

export default withRouter(connect(mapStateToProps)(App));

Below is Homepage.js:

class Homepage extends React.Component{

    componentDidMount(){
        console.log("component mounted");
        this.props.dispatch(fetchPosts());
    }
    
    render(){
        console.log("Homepage reached");
        if(this.props.userName==='') return <Redirect to="/login" />
        return(
            <div className="container-fluid m-0" style={{paddingTop: '100px',paddingLeft: '0',paddingRight: '0'}}>
                <div className="row m-0">
                    <div class="col-md-1"></div>
                    <Main userName={this.props.userName} />
                    <Leftside userName={this.props.userName} />
                    <div class="col-md-1"></div>
                </div>
                
            </div>
        );
    }
}

const mapStateToProps=(state)=>({})

export default connect(mapStateToProps)(Homepage); 

And below is the reducer:

export const userState=(state={isLoading: true,isLoggedIn: false,userName: '', photoURL: ''},action)=>{
    switch(action.type){
        case 'LOGIN': {console.log("reached");return {...state,isLoading: false,isLoggedIn: true, userName: action.payload.userName, photoURL: action.payload.photoURL, email: action.payload.email}}
        case 'LOGOUT': return {...state,isLoading: false,isLoggedIn:false,userName: '',photoUrl: ''}
        case 'CHANGE': return {...state,isLoading: false}
        default: return {...state}
    }
}

Basically what is happening is that initially when the app opens, this.props.userName is empty and hence Loading component is loaded. Once the firebase returns the user details, they are dispatched to redux reducer. When this happens, state.userState.userName becomes available and Homepage is mounted. As expected, its componentDidMount method runs and posts are fetched and dispatched to the redux store( posts are stored in redux store). But then suddenly Homepage unmounts and mounted again and consequently, componntDidMount runs again. So, in total, there are two fetchPost requests.

I do not understand this behaviour. I have read that componentDidMount runs only a single time.

Please help me to remove this bug. Thank You!