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.
*You will need to be a Subscription Administrator to view ALL Milestones using the WebService API for Agile Central*