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Everyone connecting to the RDS has Office 365 Premium. In that case there is no reason to purchase a license for the Server. Just install via the O365 ODT using Shared Activation. Of course, if it's 2019 it's not going to work on a 2012 Server, making this all moot. On RDS servers we need the Windows Search Service, which is part of the File Services. The Windows Search Pre-Installation Considerations article from Microsoft TechNet. Prevent indexing of certain file types, Enabled; Prevent indexing public folders, Enabled. Weekly Security Audit for Office 365.
Summary
On a computer with Remote Desktop Service (RDS) enabled, it is possible to install multiple versions of Office. However, this is not a supported Office configuration.
To get the computer into a supported configuration, you can either disable RDS or you can uninstall versions of Office, leaving only one version installed.
More Information
Please see the following articles for additional information about recommended and supported methods for installing multiple versions of Office on the same computer: