Symptoms

We're getting below error while starting K2 service -
Item has already been added. Key in dictionary: 'Something.Something2.Something3.SQLUM.SqlMembershipProviderProxy' Key being added: 'Something.Something2.Something3.SQLUM.SqlMembershipProviderProxy'

We are using a custom User Manager and 'SqlMembershipProviderProxy' is the class for the custom user manager. The setup is working in other environments but not working only in DEV server.
 

Diagnoses

Noticed that there was another custom 'Something' assembly in the 'C:Program Files (x86)K2 blackpearlHost ServerBinsecurityproviders' directory that was not needed.
 

Resolution

Moving this other 'Something' assembly out of the 'C:Program Files (x86)K2 blackpearlHost ServerBinsecurityproviders' directory resolved the issue.