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I believe you're referring to the Atlassian Companion app, which can be used to edit files in Confluence.
There's not a way to prevent simultaneous edits within the Companion App, but we do have a feature request below:
In addition, we have two similar requests to improve the functionality, since, at the time of this post, changes are not currently merged when a document is edited at the same time:
Please do vote on the requests above that apply to your situation. You can watch them in order to be updated of any progress.
Thank you for your understanding, and do let me know if you have any questions.
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