You can use Distinct https://learn.microsoft.com/en-gb/azure/data-explorer/kusto/query/distinctoperator
so add a last line, like
| distinct ApplicationName
However in your case you may need to summarize the resuklts
| summarize by ApplicationName, UserPrincipalName
https://learn.microsoft.com/en-gb/azure/data-explorer/kusto/query/summarizeoperator
Also check that {UserName:lable} is correct, :label is the normal spelling - some workbook templates had this incorrect and the copies of those were also wrong .
SigninLogs
| summarize make_set( AppDisplayName) by UserPrincipalName
| where UserPrincipalName == '{ name of the user from User drop down}'
| mv-expand set_AppDisplayName
| project set_AppDisplayName