ArcGIS Online allows us to create groups that allow members to update the same map. In cases where we have multiple people contributing to the contents and design of a map, there is no way to avoid two people making changes at the same time and loosing work. It doesn't even warn you when you loose work. What would fix this is if there is a check in/out feature that is common among all source control systems. Please introduce something that will allow change attempts to see if someone is current working on the same map/layer/content/etc.