Per the referenced document, 'This is due to a rare issue with Solr core rollover identified as CB-13790. This defect is scheduled to be addressed in the next product release (currently targeted for: 6.1.2). For this issue specifically there are two 'writer' cores. Normally there is only one 'writer' core, and it should be the core with the most recent timestamp.
Running the command from Step 1a in the referenced document will identify the 'writer' core, in this case there will be two: