Problem in downloading AssetBundle

I have searched this forum but didn't get the solution of my problem. Please help.

I am downloading my game scene through asset bundle. In normal cases, everything works fine as needed. But sometime, one in a five, after completing the download, error popup "Cannot load cached AssetBundle. A file of the same name is already loaded from another AssetBundle". Firstly how can this error be there if i am downloading for the first time. And because its coming at first time, i have no reference to unload that asset bundle. Please suggest what to do.

Thanks