As I wrote in the earlier reply - you should also modify the “File” target to prevent duplicate log file name
<target type="File" name="WorkflowLogFiles" fileName="${WorkflowLoggingDirectory}/${shortdate}_Execution.log" layout="${time} ${level} ${message}" keepFileOpen="true" openFileCacheTimeout="5" concurrentWrites="true" encoding="utf-8" writeBom="true" />
E.g. you could add USERNAME environment variable in the file name
fileName=“${WorkflowLoggingDirectory}/${shortdate}_${environment:variable=USERNAME}_Execution.log”
For more Nlog configuration options refer to reference:
https://nlog-project.org/config/?tab=layout-renderers
Cheers