Welcome to the world of Blender Boni.

In answer to your question, MatLib comes with Blender 2.79b. I did check my Blender 2.78a, and it's not there, so I would have to download it. Since you're getting the error message that there's 2 MatLib files, that's most likely because it already existed in your 2.79b installation.

What you need to do is go to File > User Preferences > Add-ons (see the top menu), and then on the left side choose "Materials". What you'll see is 2 Material add-ons, and all you need to do is click on the small checkbox next to the Material: Material Library (top of the list), Save User Settings, and you'll be all set. You need to do this whenever you want to initiate an Add-on, and you might want to check out the list (which IS huge), to see if there's any others you want to play with. Just choose a group of Add-ons to look through, because you'll be scrolling forever if you look at "All" of them at once.

Anyway, here's a screenshot of my Blender 2.79b User Preferences so you can see what I'm talking about.

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OK . . . Where's my chocolate?

Butterfly Dezignz