Best way is to use localstorage, not sure if my placements of setting ang getting value from localstorage are on the right spot.

const Main = () => {
    // ....

   // Here I'm forcing user to login if there's user is logged in then want to make a silent post request, But it sends request everytime on state change.
   useEffect(() => {
    getLocalStorage()
    // Check if the value of logged is true initiali will be false until the 
    // first request if made
    if (!!localStorage.getItem('logged')) {
      setModalShow(true)
    }
    if (userInfo) {
      dispatch(postRequest())
      setModalShow(false)
      // set the value when the request is finished
      localStorage.setItem('logged', true)
    }
  }, [userInfo])
  return (
    <div>Some JSX </div>
  )
}

export default Main

There is a package named redux-persist that you can save the state, for example in localStorage. You can use this package, and send post request if there is not any data in state.

Using localStorage for that purpose is pretty useful, you can save the information on post request whether it was made or not.

For a basic setup; this could be like that:

const postRequestStatus = localStorage.getItem('postRequestMade') ? JSON.parse(localStorage.getItem('postRequestMade')) : null

useEffect(() => {
    getLocalStorage()
    if (!userInfo) {
      setModalShow(true)
    }
    if (userInfo) {
      setModalShow(false)
      if (!postRequestStatus) {
        dispatch(postRequest())
        console.log('Post Request Made')
        localStorage.setItem('postRequestMade', true)
      }
    }
  }, [userInfo, postRequestStatus])

Here's a catch. As far there is information in localStorage, of postRequestMade true . The request won't be made. So some point on the site you should set any logic to clear it out where it is necessary.

Secondly, What if the request was not successful if there was an error from the server. Then, you should also consider error handling as well. As you mentioned you are using redux and I'm sure there would be Axios as well try the functionality like that:

useEffect(() => {
    getLocalStorage()
    if (!userInfo) {
      setModalShow(true)
    }
    if (userInfo) {
      setModalShow(false)
      if (!postRequestStatus) {
        dispatch(postRequest())
        // That block will take care if request was successful 
        // After a successful request postRequestMade should be set to true. 
        if (success) { 
           console.log('Successful Request')
           localStorage.setItem('postRequestMade', true)
         }
      }
    }
  }, [userInfo, postRequestStatus, success])