workloads on top of other workloads

Here in the office, one associate may not only work on his assigned project. Sometimes, he/she gets to tackle tasks other than his pre assigned tasks. Reasons could be urgent customer request or tight deadline or both. Or it could be that the previous assigned associate is currently not available to work on a certain request so another associate will have to take his/her place. Worst case scenario, previous assigned associate has resigned and is therefore deemed necessary that another engineer may take his/her project on top of his/her current workload. Case like this is crucial i may say. It will take longer lead time for the succeeding one to finish in addition to the amount of time to be accumulated for the learning curve. However, this may not be inevitable as we can't hold on people to stick with the company forever nor ask people to stick on one project for a lifetime, well rhetorically speaking. Maybe a task management software will do some magic and aid in organizing various tasks around.