Trying to create a milestone with the same name and date attributes as an already existing milestone will result in an error such as
Validation error: Milestone.name and date: Milestone '<formatted ID>: <milestone name>' already exists with the same name and date in this workspace
Searching for the milestone doesn't always return results, so it seems like the milestone may not actually exist.
Release:
Component: ACSAAS
Due to how the relationships of milestones and projects are defined, it is difficult to search for a milestone unless you are directly scoped to one of the projects where the milestone is already associated. Setting the option to search in parent or child projects has no effect on this behavior.
*You will need to be a Subscription Administrator to view ALL Milestones using the WebService API for Agile Central*