Webapp stuck with "Key duplication when adding: httpModules"

Hi,

some fixes that should improve my situation with "Key duplication when adding: httpModules" problem

GetSection under the hood is doing caching in an hashtable, which is not thread safe, so if you read section "a", and write section "b" (cache miss -> write in the hash table), the output is undefined.
Also  c (type _Configuration/System.Configuration.Configuration) isn't thread safe, so we can't fix this bug lower in the chain of class.