Users are allowed to specify a WiredTiger config string to use in collection creation.

However, the string returned by WiredTigerRecordStore::generateCreateString() can list the same field twice, one with the default value and one with the user specified value.

WiredTiger uses the latter value if a field appears twice in the provided configuration string. 

Example:

The the default 'block_compressor' is 'snappy', but if a user runs the following:

db.createCollection("myColl", {storageEngine: {wiredTiger: {configString: "block_compressor=zlib"}}})

WiredTigerRecordStore::generateCreateString() generates a config string which first specifies `,block_compressor=snappy` followed by `block_compressor=zlib`

"type=file,memory_page_max=10m,split_pct=90,leaf_value_max=64MB,checksum=on,block_compressor=snappy,,block_compressor=zlib,key_format=q,value_format=u,app_metadata=(formatVersion=1),log=(enabled=true)