Assert every instance of a json attribute Is UNIQUE

One way I did this, to assert uniqueness across multiple paginated replies is to make use of a properties step. This way, as I assume you will page through the request multiple times, you can keep an accurate count of how many times a value appears.I had to do this to ensure we were paginating through every object in the cache, and had to keep a running total of how many times any one unique identifier came back. I had 20,000 objects and could only pull 200 at a time, so I kept a running properties step that contained the unique ID and how many times it appeared. If pagination worked as expected, I ended up with 20,000 objects with each having a count of 1. Should be similar to what you are trying. 

Something similar to this hastily written psuedo code:

 
 // Set up a property step to contain our property names and counts.
 def propertiesStep = context.testCase.testSteps["NameCount"]
 
 // get the list of DataSourceId_Name values on current page of values
 DataSourceId_Name= $['data']['DataSourceId_Name']
 
 // Loop through the idNames
 for idName in DataSourecId_Name
 {
 
	// get the property value from the properties step above.
	def propertyValue = propertiesStep.getPropertyValue(idName);
	
	// if the property has a value, increment it by 1
	if (propertyValue != null)
	{
		propertiesStep.setPropertyValue(idName, (propertyValue.toInteger() + 1).toString());
	}
	// if the property does not exist, assign it a count of 1
	else
	{
		propertiesStep.setPropertyValue(idName, "1");
	}
 }

richie ,

There is no need to create properties step.

Just verify with in Groovy script itself.

def response = context.expand( '${REST Request#Response}' ) 
def json = new groovy.json.JsonSlurper().parseText(response)
def totalNamesCount = json.data.TransporterId_Name.size()
//Below removes the duplicates and does the count
def uniqueNamesCount = json.data.TransporterId_Name.unique().size()
// If you want find which name and number of times it occurs, uncomment below block of code
/*
def list = json.data.TransporterId_Name.unique()
list.each { element ->
	log.info "$element : ${json.data.TransporterId_Name.findAll { it == element}.size()}"
}
*/
//Since you wanted all the names are unique, both the counts should be same
assert totalNamesCount == uniqueNamesCount, 'There are duplicate Transporter Names' 

Please see the above if you want to check the same?