Multiple instances of one workflow are running simultaneously on an item or document.
 

The solution will be case-by-case, but most commonly the solution to this behavior is to review and modify the workflow start conditions.

These workflow instances are started at different points in time and are not direct duplicates of each other.

The second instance of the workflow is initiated on the same item when the following conditions are met:

•     A Nintex Workflow set to initiate on Item Created and Item Modified.
•     The same Nintex Workflow being republished while preexisting workflow instances are still running.