The short answer is that Modbus supports many different ways of reaching your data. Since not all pieces of equipment support all the different ways of reaching data, we include as many ways as possible so that you have the best chance of getting the data you need off of your equipment.
If you encounter a duplicate sensor in the Critical Labs web app, make note of the Register column in the table (values 40437 and 30347 in this example):
You can see that even though they tell you the same thing, they are coming from different places.
Solving the Issue
In this situation, the best thing to do is to select both sensors and see which one returns a value. If both return a value, pick one. In the context of Critical Labs, they do exactly the same thing.