Hello everyone ,
i know that this topic is 3 years old but i will geive the answer ,because from 3 days i try to simlate 3pcs of DS18B20 in proteus - 3 days of hard web search and nothing. The simple answer is that in Proteus ISIS all sensors have a same ID`s like the first one, so to make them different you got to:
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hit right mouse button over socond sensor (BECAUSE the first one is already unique ROM Number = B8C530)
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select "EDIT PROPERTIES" and there will open a window with sensor properties on line 6 "ROM serial number" will be again B8C530 - change the value like B8C531, next one can be B8C532......,3,4,5,6 ......
And you are ready !
Sorry for my poor english 
greetings for Bulgaria !
Ivo
[moderator: added empty lines for readability]