Reformat a field from multiple rows down to one row

I want to produce a search that returns basic information about our indexes, specifically the index name, the splunk_server(s) that have the index data, and the hosts that provided the data.

Right now I have this search:

index=* | dedup index splunk_server host | table index splunk_server host

This returns a very large table where each row contains a single "host" entry:

index         splunk_server     host
index_a       server001         client001
index_a       server001         client002
index_a       server001         client003
index_a       server002         client001
index_a       server001         client002
index_b       server001         client001

What I would like is to group all of the hosts together when the index and splunk_server match.

index         splunk_server     host
index_a       server001         client001, client002, client003
index_a       server002         client001, client002
index_b       server001         client001

Adding the mvcombine option helps:

 index=* | dedup index splunk_server host | sort index splunk_server | mvcombine delim="," host | table index splunk_server host

..but, there are still many places where the index+splunk_server are the same but the hosts between these lines aren't combined.

Any ideas?