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In the Middle Ages , the Caliphate began conducting regular censuses soon after its formation , beginning with the one ordered by the second Rashidun caliph , Umar .
 Caliphate, Rashidun, Umar
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Caliphate, Rashidun, Umar