If the permissions are changed on a node that is inheriting permissions from multiple groups, duplicate entries are returned in Solr queries.
Steps to reproduce:
1) Create a folder (perm2).
2) Create two permission groups (group1 and group2)
3) Via manage permissions, add the two groups to the perm2 folder (See: 531045_fig1.jpg)
4) Create a file in perm2 called "nevermore"
5) Query the file using lucene (see: 531045_fig2.jpg). You'll see one entry returned as expected.
6) Add a group or user to the permissions of "nevermore" (see: 531045_fig3.jpg)
7) Rerun the query used in step 5. You now see two entries returned by it (see: 531045_fig4.jpg)