This issue is often seen with multiple SQL Monitor Base Monitor installs targeting the same SQL Monitor Database Repository.

 

Please run the following query in SSMS against the SQL Monitor Data Repository:

SELECT * FROM sys.dm_exec_sessions WHERE program_name = 'SQL Monitor - Repository'

 

If the results show more than one distinct host_name, there are multiple installs targeting the same database. You can resolve the issue by reducing this to one connecting Base Monitor.