Hi @umesh var
I would have said that the secondary DNS is only used if the primary is not available, but on a Windows 10 machine, I can see the DNS request being sent to the both DNS servers when using the ping command from the command prompt.
However, if the WinSock32 gethostbyname function is used from an application it only sends a DNS request to the primary server.
I don't have access to a dev environment at the moment, but the DNSQueryEx function, does support the specifying of multiple DNS servers, this might cause the double DNS requests being sent.
So I think the answer when a single or double DNS request is sent will be based on the implemetation of the name lookup function in the application. Will it cause problems, it shouldn't as both request and results will be used to the populate the local DNS client cache, and typically, the first returned result will be used. The only time it could cause inconsitencies if the the DNS servers retrun different results for the same name being queried.
Gary.