I'm going to try and explain this, but this was the weirdest bug I have ever experienced. I was on a server but this wasn't lag-related. 

My Steps:

-I created an enchanted sword named "Raining Fire" from a sword that had sharpness IV, knockback II, unbreaking III, Looting III, another sword with sweeping edge III, and a book with efficiency IV and Sharpness IV. In total, the sword was Sharpness V, Knockback II, Unbreaking III, Looting III, and Sweeping Edge III

-I was looking for the final enchantment, fire aspect, and was enchanting empty diamond swords.

-I see "looting III" in the final enchantment and decide to get another sword with Looting III.

-I click it and I get a sword named "Raining Fire" with Sharpness IV, knockback II, unbreaking III, Looting III. My 3 experience levels and lapis were gone. My previous "Raining Fire" sword was gone, and the book and other sword I used were gone. 

What I expected to Happen:

When enchanting a new diamond sword, I would get a new enchanted diamond sword with Looting III from the Looting III on the enchanting table.

What Happened:

I got a previous sword with a name and my other sword (with more enchantments and the same name) was gone. 

Steps to Recreate:

I have no idea. I have enchanted about 150 swords today and this was the first time this happened.