Differentiating between first import and reimport in the TextureImporter

Is it at all possible to differentiate between new textures and old textures that are being reimported in the TextureImporter? I'm trying to build a script that hooks into the Texture Importer and reimports textures based on user input settings after they've imported. This works fine except that the GUI window will pop up any time any sort of asset operation happens that reimports or otherwise runs the texture through "OnPreprocessTexture" in any way. Is there a way I can check within OnPreprocessTexture if the texture being processed is new to the project or not?