SYMPTOM:
A user creates a documents with two datasets that are pulling data off of the same cube. If both reports have the same prompt on the same attribute, the expected behavior is to see the prompt only once. However in Strategy Web 9.x and Strategy Mobile 9.x, the prompt is displayed once per each time it exists within an underlying report. Below, is a sample document with the same prompt in two different datasets. The prompt shows up twice.

STEPS TO REPRODUCE:
CAUSE:
This is working as designed. When the prompt is created locally in the report view filter, there is no connection between the prompt in one report and that of the other reports. Thus a prompt created within each view filter will show up that many times in the document execution.
ACTION:
Instead of creating the prompt object on-the-fly, create a standalone prompt object. Insert this prompt object into the view filters of each report that requires it. Users executing the document will no longer be prompted multiple times for the same prompt.