if any person deploy any dll with name "a" & another person also deploy with same name then it is called as dll hell problem

it can be solved by Strong Nameing the Assembly which r store in GAC.

DLL Hell: while using windows dll's you will find lot of issues may be because of backward compatibility or may be because of uninstallation of existing software. This kind of errors associated with windows dll is called as DLL Hell. Ex1: Dll Hell due to backward compatibility. Assume you are using a Rolling balls game which is using a ScoresHistory.dll which will be useful in showing the previous game score as well as Highest game score. Assume you are trying to install another Ice Hockey game which also uses ScoresHistory.dll and while installing this game it has installed new version of the ScoresHistory.dll which is having some extra methods and some of the methods present in the old version is not available in the new dll. While installing the new dll windows o/s will show a message saying “already a dll with the name ScoresHistory.dll is existing, do you want to replace existing dll”. When you click on yes old dll will be replaced with new dll. Now when you try to use IceHockey game will work properly but when you try to use Rolling balls game it will throw an error because IceHockey game is trying to call a method which is not existing in the new dll. for similar type of question in c# you can refer this link also c# interview questions

It is a name referred to an issue.

It is a term used to refer a common issue found in COM technology where the newly added DLLs with the same name caused compatibility issues. The assemblies in .net resolved this issue by versioning.