Shrink operation error message text "must be a" is doubled.

[raulk89]

Elasticsearch version (bin/elasticsearch --version): 6.5.3

Plugins installed: [ingest-attachment]

JVM version (java -version): 1.8.0_191

OS version (uname -a if on a Unix-like system): 3.10.0-957.1.3.el7.x86_64

Description of the problem including expected versus actual behavior:

Steps to reproduce:

Please include a minimal but complete recreation of the problem, including
(e.g.) index creation, mappings, settings, query etc. The easier you make for
us to reproduce it, the more likely that somebody will take the time to look at it.

PUT my_source_index
{
    "settings": {
        "index.number_of_shards" : 4
    }
}

PUT /my_source_index/_settings
{
  "settings": {
    "index.routing.allocation.require._name": "dc1-etelastic", 
    "index.blocks.write": true 
  }
}

POST my_source_index/_shrink/my_target_index?copy_settings=true
{
  "settings": {
    "index.routing.allocation.require._name": null,
    "index.blocks.write": null,
    "index.number_of_shards": 3
  }
}

And the error is as follows, note that "must be a" is doubled.

"type": "illegal_argument_exception",
"reason": "the number of source shards [4] must be a must be a multiple of [3]"

Provide logs (if relevant):

[elasticmachine]

Pinging @elastic/es-core-features

[jasontedor]

Thanks for the report @raulk89.