I've migrated my store from version 1.9 to 2.1.1 using a tool.
However all the old products are appending html.html at the end of the URL. I basically truncate the url_rewrite table and saved all the categories. That created new URLs.
It looked good for few days, but now slowly I am noticing that some URLs are changing again to html.html.
This is not good for any business. It looks like when I create a new category or edit an existing category, all the products are being affected.
The new products entered into 2.1.1 are not affected by this bug — only the old ones I migrated from 1.9.