Duplicate records can arise in various ways.
The two most common are performing multiple similar searches in the same database, and importing results, and performing similar searches in two different databases and importing results (databases can overlap in their article coverage).
EndNote makes it easy to remove these duplicates.
Note. If you have three or more duplicates for a reference, you will only be presented with two references at a time to compare
For example, you have duplicates 1, 2, and 3.
You are presented with duplicates 1 and 2.
If you decide to keep duplicate 1, you will then be presented with duplicates 1 and 3.
If you have a lot of duplicates, you can delete them all at once.