Does this plugin duplicate payment after merge?

No, it will simply sum the total and will keep each item separately as it should be. Consolidation feature has been tested carefully.

Will the user audio receive notification of new order being created or do we need manually send?

You need to manually notify him by email but in his account, he will be able to see those newly created orders under orders page.

P.S. Maybe in upcoming versions i will add that feature to enable/disable email notifications on split action.

Bro you should also update name… In google search you would get more finds if name was not just splitter… something like WooOrder Manager – Split & Merge Orders

You are right, in fact basic feature was split action only and later other features are added. I will think about it and will take appropriate action.