EndNote desktop offers greater functionality for carrying out systematic reviews. If you only have access to EndNote online, there are some features that are useful for systematic reviews:
EndNote offers an easy way to find, compare and remove duplicate references.
Organize > Find Duplicates
EndNote online will allow you compare records and select the one with the best information. You can then delete the other references. You may wish to add them to another group first if you need to keep a copy of them.
To delete, rename and share groups of references, choose Organize > Manage My Groups