ConnectionString has already been added（虚拟目录的web.config节点不继承上级目录）

A的设置

<httpModules>
          <add name="SiteCache" type="SiteCache"/>
        </httpModules>

B的设置(不用的项，清除)

<httpModules>
     <clear/>
</httpModules>

如果A.B都用到

<appSettings>
  <add key="app" value="" />
</appSettings>

B运行时会提示app已经加载，此时可以

<appSettings>
  <remove name="app" />
  <add key="app" value="" />
</appSettings>

或者：

<appSettings>
  <clear />
  <add key="app" value="" />
</appSettings>