Thank you for your interest in NodeXL!
The Merge Duplicate Edges function takes every instance of an edge (some pair "AB") and counts the number of those instance and replaces them all with a single edge ("AB") and a "weight" equal to the count of edges.
In the Twitter data import you can set the importer to collect multiple instances and types of edges.
NodeXL currently can optionally create edges for each "Follows", "Replies", and "Mentions" event found in the data.
These events can happen with various frequencies. "A replies to B", for example, can happen many times, while "A follows B" can only happen once.
I usually do not Merge Duplicate Edges when I look at Twitter data because I like to filter the network based on the different types of edges present.
Our Merge Duplicate Edges feature is currently very crude. It cannot merge edges flexibly.
For example, you might want to merge all follows, replies, and mentions into three different merged edges. We do not yet do this.
You might want to sum, average, take the max, min, or median some attribute each edge: we do not do this yet.
We plan to address better support for Merge Duplicate Edges in the coming months. I suggest that some existing Excel features for merging and counting data can be applied now to get the results you need.