With the potential of multiple users working within the same sandbox view, the possibility exists that one user could attempt to make and promote a change to a Catalog, Content, or Design item that conflicts with a change another users has made to the same item and already promoted. Service Design Center is capable of recognizing when multiple changes to the same item by different users would conflict and preventing that conflict.
When a user attempt to promote a change that would conflict with another user's change, they are prompted to first sync the sandbox, which updates that user's sandbox view with the promoted and approved changes by other users.
You can also sync the sandbox you're viewing to avoid conflicts before you start to make and promote changes.