Update registration.jl

Trying to fix issue oxinabox#86 (not sure what I am doing at this point)

src/registration.jl

@@ -5,6 +5,9 @@ const registry = Dict{String, AbstractDataDep}()
 function register(datadep::AbstractDataDep)
     name = datadep.name
     if haskey(registry, name)
+        if isequal(datadep, registry[name])
+            return registry
+        end
         @warn("Over-writing registration of the datadep", name)
     end
     if !is_valid_name(name)