How do I import photos from two devices that use the same filenames?

My wife and I both have the same model of camera and I find that when we both take a lot of pictures during a day that many of those pictures will have the same filename, for example: img_3104.jpg.

We import our pictures into Windows Photo Gallery 2012 into this folder structure:

2015

   2015-11-01

      img_3104.jpg

The intention is that we have an aggregate of all pictures grouped by day regardless of the device used.

Questions:

1) What happens when the import encounters a filename that already exists? Does it:

   a) Rename the new photo (ideal)

   b) Skip importing the new photo (bad)

   c) Overwrite the existing photo with the new photo (bad)

2) If Photo Gallery is not accounting for this automatically, how do we differentiate the import from the two devices? Is there a way to add or change the filename prefix?

Thanks in advance