When an indexing request is retried (due to connect lost, node closed etc),
then a flag 'canHaveDuplicates' is set to true for the indexing request
that is send a second time. This was to make sure that even
when an indexing request for a document with autogenerated id comes in
we do not have to update unless this flag is set and instead only append.

However, it might happen that for a retry or for the replication the
indexing request that has the canHaveDuplicates set to true (the retried request) arrives
at the destination before the original request that does have it set false.
In this case both request add a document and we have a duplicated a document.
This commit adds a workaround: remove the optimization for auto
generated ids and always update the document.
The asumtion is that this will not slow down indexing more than 10 percent,
see: http://benchmarks.elasticsearch.org/

closes elastic#8788