The document already open ... do you want to close it?

Question

When I am working in visual studio. Sometime I found a message (The document already open ... do you want to close it?) and this document not open. 

 

I close my visual studio and re-open my project, Then visual studio working.But I want to a permanent solution.I can't re-open my project.

 

please tell me this solution, how to open this document without re-open my  project?

 

Advance thanks for this solution.