We are on version 6.6.0 and in one of our task we have a lot of tables, our user

Could it be possible that the user is double clicking the button?

Please see related post:
forum.appian.com/.../b-15090

It was on the Save, not on the submit button. I tried in in my DEV by double-clicking the "save" and the submit button... but it didn't get duplicated. It doesn't happen often, just wondering what could trigger this..

This issue will also impact the "Save" button since internally it acts like a type of submit. As you noticed it's not easy to reproduce that's why you don't see it very frequently but it is, like Alex mentions, the result of a double click on the Save button.

In order to avoid users doing a double click on the save button simply put this JavaScript on the "Save" event of the form:

$("div .asiToolbar").hide();

this will hide the toolbar right after the first click but will be automatically display once the form is saved preventing the issue you're seeing.

For those forms where the issue is with the submit button too just hide the submit button the "Submit" even of the form:

FormAPI.hide("mySubmitButtonID");