This is a major problem which will normally faced by new users of .net.
The main reason behind the problem is the duplicate "Inherits" name of pages.
You can easily resolve this problem by renaming the duplicate inherit names of Pages.
Let me describe this by giving an simple ex.
1) Make a new c# website.
2) By default an page default.aspx will appear.
3 Copy default.aspx and paste
4) Rename copy of default to "Default2.aspx"
5) Then try to make build.
6) It will show errors
Here, is the solution
1) Open default.aspx
2) Check Inherits attribute of page in page directive. it is "_Default" by default
3) Open default2
4) Check Inherits attribute of default2 in page directive. it is "_Default"
5) Change Inherits attribute of default2 to "_Default2"
6) Build application
Now build successfully.
This is the solution which I have explained with the help of small ex.
For any help please mail on email@example.com