The normal map will simply enhance the displacement and not double the effect at all.   However, if you're concerned about it, you can do your form sculpting and export that out as displacement, delete lower levels of subdivision and then move up a division or two to create the minutia detail to be extracted with a normal map. For example - you sculpt musculature and export that as a displacement. Then you sculpt pores and export that out as a normal map.  Personally, I wouldn't delete the lower levels - but that's me and truth told, it depends on the project.

 

People should understand that a normal map won't replace a displacement map - it's not really  intended to do that, as it's reliant on the mesh's silhouette to help drive the look it's trying to achieve and won't hold up as well under close scrutiny. A displacement will actually alter the silhouette of the object, essentially changing the mesh into something else.

 Normal maps are best left for objects in the mid-ground/background of a scene in my opinion, where using a displacement would be overkill. Displacement is best left for when you need a close up or when something is being held in the frame for a long period of time or when you need the silhouette of a mesh to alter and a morph won't do the full job.