Unable to delete snapshot "index and alias names need to be unique"

Elastic Stack Elasticsearch
1

I am having trouble deleting snapshots created in a previous version of elasticsearch.
I am currently using 2.1.2 and I have created these snapshots back in 1.7.2.

When I try to delete a snapshot using:

curl -s -XDELETE localhost:9200/_snapshot/repoX/snapY?pretty

I get the following error message:

{ "error" : { "root_cause" : [ { "type" : "remote_transport_exception", "reason" : "[cluster:admin/snapshot/delete]" } ], "type" : "illegal_state_exception", "reason" : "index and alias names need to be unique, but alias [indexZ] and index [indexZ] have the same name" }, "status" : 500 }

This seems to be due to the restriction that I think was introduced in 2.0.0 that index and alias names must be unique in a cluster.

I need to delete these snapshots because I am running out of disk space.

Is this a bug? Did anyone run into this already? How can I fix it?