Does your website template support Bootstrap 3? The frontend modals are all using Bootstrap modals.
Often the Joomla template create conflict. I have similar issue previously where my yootheme template has define a modal style.
Please share your link so I can check.
I re-read your post and i realize you mean that when you open the modal it show previous page before going to the original page.
It has nothing to do with js conflict at all. I have issue when I hosted my site in 1&1 before because i have very slow site. AWhen i move to siteground it works normally and speed is normal.
It show the previous page when it takes time to load the page. That is how I fix my issue before.
im already using siteground and the problem still persists.
im not yet sure but i think it has something to do with the modal content of bootstrap.
i'll update this post when i have the solution.
With the current installation every time a user experiences slow server or internet connection he will definitely see the last modal content. This might be a problem for new user(clients/buyers). We might lose clients because of this confusion. i dont think it would be professional to add a note saying "if your form hasnt appeared please wait for it, its normal"