Under the tab My References.
In order to remove...
a reference from a group (the reference is present in the [Unfiled] tab):
a reference from the library and from all groups:
Duplicates (references in double):
Using groups makes it easy to share references and create bibliographies.
A reference can be copied into more than one group, but will always remain in the master list (My References).
Create a group from My References
Create an empty group
It is possible to sort the references according to several criteria from the Sort by drop-down menu :