duplicated users

duplicated users

Hello, when I create Requests and assign them to a user which is associated to an account it happens the following:
With the first request, everything is OK. With subsequent requests, user is duplicated an unassigned to any account, so, the requests are not assigned to that account. If afterwards I edit manually the assigment it seldoms assign to the "UserXX from AccountXX" or to "UserXX".