Typical mental model suggests that submit button is at the end of the form. Should I go with button at top as well as bottom. since in our application we put action items on a sticky bar at top at top. But our user are not able to follow the submit button, we tried activating once the form is complete. Any suggestions? Is it ok to have a 2 submit button.
It's better to have submit button at the bottom. There is no point having two buttons on same form. The bottom button will help user to fill out all the necessary details present on the form as it is scrollable.
Reminds me of an nngroup article on four iOS Design Rules to Break. They recommend placing a submit button at the bottom of the page (you'll find the rationale in the article):
Display the form Submit button (or equivalent) under the form fields rather than at the top of the page.
Its ok to have 2 submit buttons if the form to be filled is long and requires scrolling. If you have a sticky button you don't need to repeat the button.
Either way, having the button at the end of the form is better as it improves the chance of the user filling all the required fields before hitting submit. or you will have the users getting error messages when required fields were missed out
It's good to have submit buttons in extra places in some cases. As Ameen says putting one at the top might suggest to the user that all they see on the screen is all they have to fill in which may result in validation errors. This is a lot more likely on create.
There is a good case for it though, which is editing complex forms. In a CMS for example, you might have the most likely to be edited fields near the top of the form and in that case a submit at the top (or fixed to the side) is much more convenient as the user doesn't have to scroll right down the form to submit it, past all the fields they rarely edit.
Remember that enter also submits forms, whether the button is visible or not - do your users do this?
Another thing you can do with a long form is break it down into sections, each of which is submitted in an easy to digest chunk. You'll notice this format used by sites like Quora or Facebook when you sign up and they ask for a variety of quite complex information about you. Then when you edit only the appropriate bit of the form is loaded and submit is not too far out of reach, even if the very common pattern of one button at the bottom of the form is all that's used.