Why do Two Instances of MonoDevelop Open when I double-click a CS file in Inspector?

I am using a Mac and have multiple instances of Unity on my computer (though not open at the same time).

Currently using Unity 4.6. Unity is set to use the Built-in Monodevelop editor.

It hasn't been a problem until recently.

The problem may have started when I set the file association for .cs files on my Mac to open with Xamarin. I have since deleted Xamarin and set .cs files to open with Monodevelop (the same one as the Unity version I am working in uses).