Page is currently taken

A typical pattern would be to consider a Record Summary page as Read Only, and to provide a Record Action to allow users to update the record's data. A Record Action will start a process and there are two different ways to prevent other concurrent updates:

• you can make the visibility of a Record Action dependent upon whether there are any instances of the same process in flight (you would need to use a Process Report to determine this). Note that there is a small window of opportunity for two users to start the same process where the Process Report will not yet return the necessary data
• you can make use of a Locking solution. The one I have used in the past is this: https://community.appian.com/b/appmarket/posts/pessimistic-locking. This is by far the better, more secure scalable solution, which can be re-used across all of your applications.