This type of issue indicates that in the activity diagram there are several signals with the same name that are all active (hold a token). Therefore, there are several activity steps which are triggered by this identical signal. This case can cause non-deterministic behavior at runtime and, thus, is not allowed.
An example of this type of error is depicted in the following figure.
A token flowing through the fork node is duplicated. One of them rests in the left hand-side perception MESSAGE and the other in the right hand-side. This means that when a signal named MESSAGE arrives, two activity steps from the two perceptions are triggered:
Depending on the behavior of your application, there are several possible solutions. For the example above, two alternative constructions as shown in the following figure fix the error.
Note that these two forms are different. In 1, operations display and process receive the same object of type
String carried by signal MESSAGE. In 2, these operation receive different object.